The pair will leave Cornwall today on a tandem to try to break the current record of 51 hours 19 minutes for the 847-mile journey.

Former rower Cracknell, who won gold medals as a member of the coxless four in the 2000 Sydney and 2004 Athens Olympics, approached Romero, three-time Olympic champion Bradley Wiggins and world ironman champion Chrissie Wellington to accompany him on the trip.

Cracknell said: "It’s testimony to the appeal of the route that, despite having to ride with me, all three were interested.

"Brad had previously suggested doing something similar together but would be busy going toe to toe with Lance Armstrong and Alberto Contador in the Tour de France.

"Chrissie has her [world] title to defend in Hawaii in October.

"Rebecca, meanwhile, was taking a year out after her success in Beijing [where she won gold in the individual pursuit, having won a silver medal for rowing in 2004].

"She was just getting back into training – perfect.

“'What better way to get some miles back in the legs?' I reasoned."

Cracknell has previously rode across the Atlantic and skied across the South Pole, both expeditions he undertook in partnership with television presenter Ben Fogle.

The current record Land's End to John O'Groats record was set by Andy Wilkinson and Lynne Taylor in 2001 and Cracknell and Romero plan to ride the route without any rest.

Cracknell said: "By attempting to ride it continuously, I believe we will be tested mentally and physically in a way we never have been before, and that includes the pressure of the Olympics.

"After our final training ride – 10 hours through the night – we were left genuinely scared, wondering how we could possibly do that for at least another 40 hours.

"The satisfaction comes from asking your body and mind that question and finding out if you can answer it."

Romero said: "It's very different to riding a normal bike.

"It's like the difference between driving a nippy car and a great big lorry.

"It's great downhill because of the weight, but not so good uphill.

"The ultimate nightmare combination is if it rains, so the road is slippy, it's dark, so we can't see, and we're struggling to stay awake.

"It adds an element of danger."