Leading with 700m left, Jonny, 26, began to weave over the road in hot and humid conditions in Cozumel.

Third-placed Alistair, 28, caught his brother, propping him up for the final few metres before pushing him over the line in second place.

They were overtaken by South African Henri Schoeman, the eventual winner.

Victory in Mexico would have given Jonny the world title, but second place left him just four points behind Maria Mola.

The Spaniard was fifth on Sunday to top the overall standings.

Jonny, Olympic silver medallist at Rio 2016 and a bronze medallist at London 2012, collapsed to the ground the moment he crossed the finish line.

He required treatment but later tweeted he was OK, with a photo of himself lying in a hospital bed on a drip.

Alongside a video clip of the finish of the race, he also wrote: "Normally when you have had too much to drink. This time it was the opposite #ouch"

Alistair said it was "a natural human reaction" to come to the aid of his brother, adding: "I'd have done the same thing for anyone in that position."

He added: "I wish the flippin' idiot had paced it right and crossed the finish line first.

"He could have jogged that last two kilometres and won the race.

"You have to race the conditions. I was comfortable in third. I raced the conditions, I took the water on, made myself cool and I was alright."

Schoeman, who finished fourth in the overall standings, won the race - consisting of a 1.5km swim, 40km cycle and 10km run - in one hour 46 minutes 50 seconds, with both Brownlees 18 seconds behind.

The International Triathlon Union dismissed the Spanish Triathlon Federation's appeal to disqualify Jonny, because "athletes can receive help from another athlete".