Jane Tomlinson has run a series of gruelling races

The event,which is taking place in Nice on Sunday, involves a 4km swim, 120km bike ride and 30km run.

It is the 40-year-old Leeds mother's latest push towards her target of raising �1m for cancer charities.

The competition is expected to take around 11 hours and covers a distance three-times longer than a triathlon.

The swim, in the sea, is equivalent to 160 lengths in an average pool.

There are no guarantees in life as we know only too well. Mike Tomlinson

The cycle leg includes three mountain passes with three "haute" category climbs - which are too high to grade.

But Mrs Tomlinson has experience of tackling such ascents having climbed the 1,909m Mont Ventoux in Provence on a tandem cycle with her brother Luke Goward.

She vowed never to complete another challenge following her "Rome to Home" challenge in June, but has decided to carry on to raise as much money as possible.

Her husband Mike, 43, said the family were all nervous about the event.

Huge risk

"The bottom line is we don't know if Jane can manage this. For the first time in two years our confidence level is less than 50%.

"Jane and I are very tired and it will be a huge relief when we get to November and she can retire. We never envisaged the appeal would continue for this length of time with the pressure being so relentless."

Mr Tomlinson said Jane's appeal had raised about �925,000 so far with about �7,000 coming in every week.

"There are no guarantees in life as we know only too well. Jane though is taking a huge risk on Sunday and the money raised will be minimal compared to those risks," he added.

"This is a personal challenge for Jane to become the first cancer patient to compete in an event of this length. Let's hope everyone is smiling on Sunday night."