Jonny looked on course to claim the overall world title as he led comfortably heading into the final kilometre of the 10km run before the heat started to take its toll and he started to weave across the course.
As South African Henri Schoeman passed the stricken Brownlee to take gold, third-placed Alistair put his arm round his brother and all but carried him for the final 100m before pushing him over the line.
The result meant Spaniard Mario Mola, who finished in fifth, took the title by four points but that was the least of the Olympic champion’s worries as his brother received medical attention at the finish line.
Alistair, who suffered a similar fate in a race at Hyde Park in 2010, said: “If it happened to anyone I would have helped them across the line because it is an awful position to be in.
“Obviously it was a natural human reaction to help my brother but to be honest I would have saved anyone. It’s dangerous for someone to conk out (close to) the finish line. It’s as close to death as you can be in sport I think.”
Not that Alistair was all sympathy, with the two-time Olympic champion adding: “I wish the flipping idiot had just paced it right and won the race. He could have jogged the last two kilometres.”
Jonny Brownlee appeared in good spirits after receiving treatment, tweeting from his hospital bed.
Alongside a video of the dramatic end to the race, he wrote: “Normally when you have had too much to drink. This time it was the opposite #ouch.”
He posted a picture of himself giving a thumbs-up sign as he lay in hospital and a picture of his runners-up trophy, also paying tribute to his brother by saying: “Not how I wanted to end the season, but gave it everything. Thanks @AliBrownleetri, your loyalty is incredible.”
He added: “Well done @mariomola and @H_Schoeman. Also thanks to the @worldtriathlon and @BritTri medical care. I couldn’t have been in better hands.”
An appeal by the Spanish Triathlon Federation to disqualify Jonny Brownlee for accepting assistance to get over the line was dismissed by the International Triathlon Union, citing a rule which states athletes may accept help.
And in an interview on the ITU website, newly-crowned champion Mola admitted the dramatic circumstances of his victory took some of the shine off his success.
Mola said: “First of all this was not how I wanted to win the World Championship. Luckily I was able to be in the top five considering Jonny didn’t win the race.
“To be honest I was already considering that the role of AIistair was to help Jonny. Considering that, I thought, well, I’m going to be second again, but at least I will fight to the end.
“And when I heard Jonny isn’t feeling well, I thought, ‘I need to fight to the end’. We want everyone to be safe after the finish line. It’s not the way I wanted it, but that’s triathlon.”