The Boat Races: William Warr set to face ex-Cambridge team-mates with Oxford

There are supposed to be only two options. In or out. Cambridge or Oxford. You can't be both.

In the long history of one of sport's most enduring rivalries, just two men have crossed the line.

Make that three.

When the the 163rd Boat Race gets under way on the river Thames on Sunday, William Warr will be going up against his old team-mates, rowing for Oxford against his former Cambridge team.

"It hasn't been easy. It was a decision I had to make, but guys I was really close with now barely speak to me any more," he told BBC Sport.

"Some have said they really hope I lose, that they completely disagree with what I'm doing, which I understand. It is a very strong bond.

"But life does go on. You need to think about your career - we are students, sometimes people forget that - and the research I am doing can help save lives, so to not go and do that because of some old rivalry would be selfish."

• Read more: The Beeb and the Boat Race

Warr, 25, rowed for Cambridge in the 2015 event. They lost, as Oxford claimed their 11th success since 2000.

But Cambridge did win last year - without Warr, who is now doing a PhD at Oxford.

He knew it was the only place where elite rowing could live alongside his field of study. And he knew it when he was still on speaking terms with the Cambridge fold.

"I came to Oxford to do this PhD on how to prevent chronic disease in some of the poorest parts of the UK," he explains.

"I also want to go to the Olympics in 2020, so the only way to combine the two aspirations was to come here.

"I spoke to Cambridge's president, we talked through the options, laid everything out, and really it was only way to go."

It could have been worse, you might say. Warr was only at Cambridge for nine months. Nine months is a long time in comradeship and toil, but the silent treatment will feel like a price worth paying if Oxford slide through Sunday's 6.8km Championship Course the quicker. Especially because of the history involved.

The Dark Blues (Oxford) trail the Light Blues (Cambridge) by 79 victories to 82 since the race began, in 1829.

But nearly two centuries on, there is no suggestion of inside knowledge tipping the scales.

"The Oxford people aren't interested in knowing what Cambridge are doing, and nor would I tell them anything," he says.

"They trust the coach, Sean Bowden, and they've been very successful over the past 15 years. I think they know enough not to worry too much.

"The two coaching programmes are slightly different on techniques, but there are similarities, and we train pretty much the same hours at both Oxford and Cambridge. The weekly timetable is really similar.

"But it will be a bit strange for me on the start-line. Because it's a special race."