Crowds watching the London Marathon, including members of the Royal Family, have watched on in disbelief as a runner pulled a fellow competitor over the finish line after he collapsed just metres from the end.

David Wyeth was struggling to stand up as he approached the finish line during Sunday’s marathon, falling over less than three hundred metres away.

Matthew Rees came around the final corner of the race and noticed David collapse and decide to help.

Matthew Rees helped David Wyeth finish the London Marathon. (BBC)

Matthew Rees helped David Wyeth finish the London Marathon. (BBC)

“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 told 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.”

Mr Rees said it didn’t matter he was so close to the line himself, he simply wanted to help his fellow competitor.

“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,” he said.

“So I put his arm around mine and we walked to the line.”

Crowds, which included the Duke and Duchess of Cambridge and Prince Harry, cheered on the pair, who was quickly assisted by race officials.

The Duke and Duchess of Cambridge and Prince Harry cheered on runners in the London Marathon. (AFP)

Mr Wyeth said he urged Mr Rees to run on and was overwhelmed when he stayed to help.

“For someone just to stop their own race to help you, that’s such a decent thing he did.”

The Duke and Duchess of Cambridge and Prince Harry cheered on runners in the London Marathon. (AFP)

The Royals were also captured handing out water to runners at the 35 th kilometre of the race and high-fiving competitors to encourage them to the end.

Prince Harry showed his joking side by using a giant foam finger to point out what way they should be running.

The trio was there to support Team Heads Together, which raises awareness of the prevalence of mental health conditions.

Prince William hugged a runner competing in the London Marathon. (AFP)