Fresh questions over Mo Farah's relationship with his banned former coach Alberto Salazar have been raised in a new BBC Panorama investigation.
Documents show Farah repeatedly denied to US Anti-Doping (Usada) investigators he had received injections of the controversial supplement L-carnitine before the 2014 London Marathon.
Farah later changed his account to Usada investigators, saying he had forgotten.
The documents also reveal how a UK Athletics official was dispatched to Switzerland to collect the legal supplement from a contact of Salazar's.
Emails obtained by Panorama show how UKA officials had initially expressed concern about whether the injection was safe and within the "spirit of the sport".
The Panorama programme Mo Farah and the Salazar Scandal will be screened on Monday, 24 February. It also reveals new allegations about Salazar.
Salazar ran the Nike Oregon Project - home to British four-time Olympic champion Farah from 2011 until 2017.
In 2015 a Panorama investigation, in collaboration with US website ProPublica, first revealed allegations of doping by Salazar, the coach widely credited with helping turn Farah into Britain's greatest athlete. The programme sparked a Usada investigation, resulting in Salazar being given a four-year ban from the sport by a panel of US arbitrators in October 2019.
Salazar rejects the findings and is appealing against the ban.
In a statement he said: "The panel made clear that I had acted in 'good faith' and without 'any bad intention to commit the violations'."
Two of Salazar's violations relate to using a banned method to administer an infusion of L-carnitine, a legal supplement.
L-carnitine is a naturally occurring amino acid, which, if injected straight into the bloodstream, some research suggests could help speed metabolism and boost athletic performance.
Infusions or injections were permitted within World Anti-Doping Agency (Wada) rules provided the volume was below 50ml every six hours.
In 2014, Farah finished eighth in his first London Marathon. Three years later, when the Sunday Times reported that he had received an infusion of L-carnitine, the Digital, Culture, Media and Sport select committee inquiry Combatting Doping in Sport called Farah's team before it to explain.
Dr Robin Chakraverty, then UKA's chief medical officer, said "an injection" of L-carnitine had been a joint decision between him and UKA's head of distance running Barry Fudge, taken after research, considering the risks and possible side-effects.
The committee was assured the volume was 13.5ml, well within the allowable limit, though Dr Charaverty failed to record it. There is no evidence any rules were broken.
Farah finished eighth at the London Marathon in 2014
Panorama's evidence sheds new light on the situation and raises questions about Salazar's influence.
Emails between UKA officials in the days leading up to the marathon reveal their concerns about giving the injection.
On 6 April 2014, Fudge wrote: "Whilst this process is completely within the Wada code there is a philosophical argument about whether this is within the 'spirit of the sport…'."
He added: "Although Alberto and Mo have expectations about doing this, we are not at a point where we… can't pull out."
Former UKA performance director Neil Black admits "a degree of discomfort".
He wrote "… should we really be trialling this process so close to the London Marathon? ... That's before we even think about the spirit of sport."
Dr Chakraverty seemed concerned about possible "side-effects."
He wrote "… it would have been better to have trialled it in someone first."
"I understand [Salazar] is keen but… we should be asking him to follow this advice."
A decision was taken to go ahead. But there was a problem: the concentrated form of the L-carnitine supplement they wanted could not be sourced in the UK. That is where Salazar comes in.
Salazar introduced Fudge to a contact of his in Switzerland who was able to order batch-tested L-carnitine in the form needed.
And so Fudge jumped on a plane to Switzerland, met Salazar's contact and collected a package of injectable L-carnitine and brought it back to London.
There was not time to trial it on anyone to make sure it had no side-effects. Just two days before the race, on 11 April, in Farah's room within The Tower - the official London Marathon hotel - the L-carnitine was injected into the arm of Farah by Dr Chakraverty.
At the DCMS select committee, Dr Chakraverty referred to "an injection". In fact four injections were given to Farah, spaced over two hours through a butterfly needle, with Salazar, Fudge and Black looking on.
Panorama understands other elite British athletes racing that day were not offered the same treatment.
Toni Minichiello, who coached Olympic heptathlon gold medallist Jessica Ennis Hill and sits on the UKA members' council, told Panorama: "That's pretty damning. I'm shocked. Barry Fudge in that instance has to explain… what was your logic for doing that?
"And you're an employee of UK Athletics, so UK Athletics, why would you allow one of your staff to do that?"
Damian Collins MP, then chair of the DCMS select committee, said there has been no mention of this level of concern, or trips to Switzerland, when Dr Chakraverty and Fudge appeared before his committee.
"I don't think we did get the full picture because what, I think, comes out of those emails is that this wasn't a routine thing," Collins said.
Asked if Salazar had been directing all of this, Travis Tygart of Usada told Panorama: "[UKA] were absolutely in concert [with Salazar], there's no doubt about that.
"I think it's the lengths that people who want to win and are incentivised to win will go, if they have the money and the resources to do it."
When athletes are drug-tested, they are required to list all medications and supplements they have taken within the past seven days. Farah was tested six days after the injection - 17 April 2014. Despite listing a number of other products and medicines, he failed to record L-carnitine on his doping control form.
A year later, as part of their probe into Salazar, investigators with Usada flew to London to interview UKA officials - and Farah.
Farah was questioned by Usada officials for nearly five hours - and Panorama has obtained a transcript of that interview.
Asked specifically and multiple times whether he had an L-carnitine injection before the London Marathon, Farah repeatedly denied it.
He was asked: "If someone said that you were taking L-carnitine injections, are they not telling the truth?"
Farah said: "Definitely not telling the truth, 100%. I've never taken L-carnitine injections at all."
He is then asked: "Are you sure that Alberto Salazar hasn't recommended that you take L-carnitine injections?"
Farah responds: "No, I've never taken L-carnitine injections."
He is asked again: "You're absolutely sure that you didn't have a doctor put a butterfly needle… into your arm… and inject L-carnitine a few days before the London marathon?"
Farah says: "No. No chance."
We have learned that minutes after the interview, Farah then met Fudge, who had been interviewed by Usada the day before.
Farah then rushed back in as the investigators were packing up. He changed his account.
Farah tells Usada: "So I just wanted to come clear, sorry guys, and I did take it at the time and I thought I didn't…"
He is asked: "So you received L-carnitine… before the London marathon?"
Farah answers: "Yeah."
He adds: "There was a lot of talk before… and Alberto's always thinking about 'What's the best thing?' 'What's the best thing?'"
The Usada investigator says "… a few days before the race… with… Alberto present and your doctor and Barry Fudge and you're telling us all about that now but you didn't remember any of that when I… kept asking you about this?"
Farah responds: "It all comes back for me, but at the time I didn't remember."
Mo Farah declined to be interviewed by Panorama.
In a letter, Farah's lawyers said: "It is not against [Wada rules] rules to take [L-carnitine] as a supplement within the right quantities.
"The fact some people might hold views as to whether this is within the 'spirit' of the sport is irrelevant.
"Mr Farah… is one of the most tested athletes in the UK, if not the world, and has been required to fill in numerous doping forms. He is a human being and not robot.
"Interviews are not memory tests. Mr Farah understood the question one way and as soon as he left the room he asked Mr Fudge and immediately returned… to clarify and it is plain the investigators were comfortable with this explanation."
The documents also reveal that Fudge did not initially disclose his trip to Switzerland to obtain the L carnitine.
When asked by Usada investigators how he obtained the L-carnitine, he said: "It is a prescription-based product in the UK, so we provided it."
He was then asked if he got it from Pete Julian, a coach at the Oregon Project. He answered: "No, it was a prescription-based product."
However, he returned to the interview room the next day, having been asked by Usada to provide relevant emails. Fudge told them: "I don't think I told you guys enough… I don't think I told you anything that wasn't correct, I just feel I probably should expand on it a bit more."
Fudge then told Usada that batch-tested L-carnitine hadn't been available in the UK, and that he had travelled to Switzerland to collect the product from Salazar's contact.
Collins added: "This very specific medicine was required, sourced at great difficulty, given against the initial advice of the doctor. But yet, no-one keeps any records of it and everyone decides to keep quiet about it.
"I think this is something that should be looked at in some seriousness."
In a statement Dr Chakraverty said: "I have not contravened any [world anti-doping] rules, and have always acted in the best interests of those I treat.
"The evidence I provided to [MPs] was an honest account - including an acknowledgement that my usual standard of record keeping slipped due to heavy work commitments and travel.
"The GMC reviewed this and concluded that the case required no further action."
In a statement, UKA said: "A small number of British athletes have used L-carnitine and, to our knowledge, all doses and methods of administration have been fully in accordance with Wada protocol.
"The dosage provided to Mo Farah was well within the 50ml limit permitted.
"Full and honest accounts of the process were given in all forums. Any suggestion to the contrary is false and misleading."
Salazar said: "No Oregon Project athlete used a medication against the spirit of the sport. Any medication taken was done so on the advice and under the supervision of registered medical professionals."
A fresh UKA review is under way to establish whether any mistakes were made in its handling of the Salazar episode.
Collins added: "I think it leaves UK Athletics in a very difficult position. And this seems, to me, that UK Athletics effectively… gave Alberto Salazar… sort of total control over the preparation and training of some of our most celebrated athletes with not very much oversight from people at UK Athletics as to what they were doing and whether they were acting in the best interests of either the sport or that individual athlete and that's a failing on their part."