The disease affects one in 50 children, leading to mental health problems and missing school.

"If anyone has done a cross-country [run] or a marathon - that is how it feels all the time," said Jessica, 14.

The trial, on 734 children, will use intensive online therapy sessions to adjust sleeping habits and activity levels.

It also uses a form of behavioural therapy to help children with the disease adapt the way they live.

Studies suggest one in 100 children misses at least a day of class a week because of the disease.

When Jessica - not her real name - was 10, she missed the equivalent of a whole term in one school year.

She was diagnosed with chronic fatigue syndrome (CFS) - also known as myalgic encephalomyelitis (ME) - at the age of 11.

"I can't do the things my friends can do," she told the BBC News website.

"I've missed a lot of birthdays.

"When they have sleepovers, I have to sacrifice that.

"And I can only do mornings at school, so I miss a lot of lessons."

Prof Esther Crawley, a children's doctor and from the University of Bristol, said: "This illness is devastating. About 50% of teenagers are tired, but these children are different - they stop doing the stuff they want to do.

"The first thing they drop is socialising and fun things, then they drop school, so this is very different to teenagers just being tired."
'Lost friends'

Jessica's mother said: "It has been hell, it has been depressing.

"I have lost friends, she has lost friends.

"She loves Florence and the Machine and her best friends went [to watch the band play live], it is isolating because her friends were doing what her mind wants to do but her body cannot."

Prof Crawley is leading the FITNET-NHS trial, to see if online consultations work and are cost-effective for the health service.

It is being funded by the research wing of the NHS in England, but when the results are out all the devolved health services would then decide whether they wanted to introduce it.

Trials of the scheme in the Netherlands showed 63% of the patients given therapy had no symptoms after six months, whereas just 8% recovered without it.

The scheme offers behavioural therapy sessions to change the way children think of the disease and aims to reduce the time spent sleeping and sometimes cut activity levels.

The approach regularly receives criticism from some activists who argue it treats chronic fatigue syndrome as a disease of the mind.

Prof Crawley said: "A teenager might say, 'You are just trying to change my sleep', but do you know how much biology you actually change?

"Children who come to my clinic have low cortisol [stress hormone] levels in the morning, that is why they feel so terrible; by changing their sleep, we reverse that.

"The stuff we are doing is not a pill, but it might as well be."

Lizzie Horgan, 26, from Hertfordshire, is just getting back to work after being hit with chronic fatigue syndrome in 2015.

"One evening I came home feeling really unwell and passed out, I had no control over my body.

"I was initially really reluctant to take up the offer of cognitive behavioural therapy, as I assumed it was for something psychological, but actually it really helped me understand what was happening to my body.

"So I think the trial is really cool and really exciting."

The results of the study should be known in 2022.

Mary-Jane Willows, from the Association of Young People with ME, told the BBC: "It is a scandal, what other condition would you leave a child with - undiagnosed and untreated?

"And the trouble is the longer they are left, the more ill they become.

"This trial is hugely important, every day we are phoned by parents who are desperate to get treatment for their children."

Prof Stephen Holgate, from the Medical Research Council, said: "CFS/ME is an important disabling condition.

"It has been chronically underfunded for years; patients deserve high-quality research like this."