Thousands of people in the crowd – including members of the Royal Family – watched on in disbelief at the heartwarming gesture taking place before their eyes.
With only a few hundred metres left before the finish line, competitor David Wyeth was struggling to stand and looked like his afternoon was about to end prematurely. Enter fellow competitor Matthew Rees.
Coming around the final corner of the race, Rees noticed Wyeth collapse and decided to help. Watch the video below to see this amazing act of sportsmanship:
"I saw a runner struggling, his legs were collapsing beneath him and every time he tried to get up he kept falling back to the ground," Mr Rees said in an interview with the BBC.
"I went over to him and said c'mon you can do this and tried to gee him up but every time he tried to get up I realised that he wasn't going to make it."
"I said c'mon we can do this, we will do it together, we'll cross the line together."
"We both wanted to finish the race and the sub-three was on for both of us if we could just get to the line," Rees said.
"So I put his arm around mine and we walked to the line.
Mr Wyeth was apparently overwhelmed when he came-to after the race at what had just happened.
"For someone just to stop their own race to help you, that's such a decent thing he did," said Mr Wyeth.