He had endured a gruelling three-and-a-half-hour run and was barely a mile from the finish line, almost within earshot of the roar of the crowds on the Mall.
But Jake Halliday, 28, who was running the London Marathon for a dear friend diagnosed with cancer, was suddenly plucked from thethrong by a steward and asked to leave the course.
His bib had fallen off and competitors cannot go on without an official number, the Telegraph reports.
A short time later, however, a shaven-headed and bare-chested participant jubilantly crossed the finish line wearing Mr Halliday's number, later posing for photographs and triumphantly kissing his medal.
He was also pictured drinking champagne and blowing kisses to the crowd.
The unidentified imposter is thought to be a spectator who picked up the trampled bib and took it upon himself to complete the last part of the course in Mr Halliday's name.
Jake Halliday earlier on in the race. Photo / Marathon Foto
London Marathon organisers confirmed they had launched a "full investigation" into the incident, which includes checking CCTV images of the relevant section of the route and interviewing several people including Mr Halliday, who works for an asset management and investment company.
Peter Mowbray, 51, from Blackpool, Lancs, described the debacle on Facebook after hearing rumours at the finish line that a runner had been removed from the course and was unable to receive his medal because he was not wearing a number.
He then chatted to other runners, who identified Mr Halliday as the victim.
Mr Mowbray, an operating assistant at Blackpool Victoria Hospital, said: "I couldn't believe this so we went on the Virgin London Marathon website and you can clearly see Jake's number and you can clearly see the pictures of Jake are completely different to the guy with his number at the end.
"Running a marathon takes over your life, you think about training, nutrition, avoiding injury, a huge amount dedication, and that's before you think about raising money for your charity.
"For this man to take all the glory and credit for someone else's hard work and endeavour really makes me question human life at the moment, it really does.
"To see this man smiling and postulating with someone else's medal made me personally very very angry. I felt very sick at first."
The hoaxer poses with 'his' medal. Photo / Marathon Foto
He said he wanted to share it on social media because he felt people should be aware of what had happened.
Mr Halliday was running for blood cancer charity Bloodwise and was last tracked with 1.4 miles to go, on course to finish the marathon in under three and three quarter hours.
He was pictured at several points along the route running alongside a friend from Edinburgh, Andrew Keiller, also 28, but his finish time was never recorded.
The pair, and a third friend, Chris Chisholm, 28, who is taking part in the Stirling marathon this weekend, have raised almost £50,000 for Bloodwise, smashing their £20,000 target.
The trio were fundraising on behalf of Mr Keiller's girlfriend Emma Reid, who was diagnosed with non-Hodgkin lymphoma in October 2016.
On their fundraising page, Mr Keiller described how Miss Reid had developed a cough, which was later thought to be pneumonia, before getting the devastating diagnosis.
The hoaxer kisses Halliday's medal. Photo / Marathon Foto
"She is a young, fit, healthy non smoker. It was nothing she had done, nothing genetic, just horrible rotten luck," he said.
Since then, she had endured ten rounds of aggressive Chemotherapy, a stem cell transplant and weeks of radiotherapy, resulting in unimaginable physical and mental exhaustion.
"Emma has dealt with it all with courage, grace and bravery, in a way that words won't do justice," he added.
"In recent months the condition has improved and earlier this month, 15 months on, we got the amazing news that there is no evidence of any active Lymphoma left.
"The team at the Western General in Edinburgh, from the lead consultant to the domestic staff, have been nothing short of unbelievable. (We are extremely lucky in this country to have the NHS and shouldn't forget it!)"
The trio said Bloodwise was a "wonderful charity" whose incredible work had saved Miss Reid and thousands of others.
Matt Lawley, head of sports at Bloodwise, said: "We are really grateful to the team for all the money they have raised, and we know that the London Marathon are fully investigating this."
Mr Halliday declined to comment.
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