Imagine the scene on Friday.

The men's 20 kilometre race walk is drawing to a close. The leader enters the Olympic stadium, just metres short of the finish line, a prize of $50,000 and the dream that has intoxicated him for four long years.

Exhilarated, he inadvertently breaks that wafer-thin line between a legal fast walk and an illegal slow run. Then he spots Peter Marlow.

Marlow is moving forward, arm slowly rising from his side, armed with the red bat that spells disqualification.

Not easy, being the chief race walking judge at the Olympics.

"Well, it can be difficult. I hope it won't be the worst job at the Games," Marlow told Reuters on Thursday.

"Most walks are already settled by the time the competitors get to the stadium, so it's rare to have such a problem. But yes, a sprint finish would be the worst nightmare for me."

Race walkers are allowed two warnings for "lifting" -- both their feet leaving the ground at the same time -- before they are thrown out of races.

The chief judge, stationed right at the end of races, can also disqualify walkers.

Competitors live with the risk that they will be disqualified from some events, says Marlow, but it stands to reason that the later the dreaded red bat, the greater the walker's distress.

HORROR STORIES

Race walking history is full of such horror stories, perhaps none quite as heart-breaking as Bernardo Segura's agonies at Sydney.

With 100 metres of the 20km event remaining, he upped his pace to get clear of two rivals to win the closest finish in Olympic history.

He donned a flag and hat and was on the phone to the Mexican president while being interviewed on television when he was told he had been disqualified in the final metres.

"I think the president had also just told him that they were declaring the day a national holiday," Marlow reflected.

Few who watched Segura's very public anguish will ever forget those images, nor indeed those of Australian Jane Saville in the women's 20km, who broke down in floods of tears as she received her third warning at the entrance of the Sydney stadium.

Both are due to compete in Athens.

The 62-year-old Marlow is as sympathetic as you can be. He competed at the 1972 Munich Games before becoming a judge.

"I know what it feels like," he says. "Disqualifying somebody is the last thing I want to do, but that's the job.

"If they saw me move towards them in the stadium, they would be petrified. I'm very visible by the track and they know me by sight. I know a lot of them personally.

"Lifting is simple to spot when you are an experienced judge. But for me to disqualify them right at the end, it would have to be blatant, they would literally have to run.

"I don't get upset but I don't enjoy it. The memory stays with you. You try to do it without being ruthless.

"I do apologise."