A gambler from Milton Keynes diagnosed with cancer stands to win £20,000 if he defies doctors' predictions and lives until June 2010.
Jon Matthews has already collected £5,000 by winning the first stage of his bet.
Doctors diagnosed him with mesothelioma, a form of lung cancer, in April 2006, and told him he had only months to live.
In 2007, he approached bookmakers William H
ill and asked them if he could place a bet that he would survive the next three years.
They agreed and Mr Matthews wagered £100 at 50-1 he would live to see June 1 2008.
He put another £100 on at 50-1 he would still be alive on June 1 this year and made a third bet, again of £100 but this time at odds of 100-1, that he will make it to June 1 2010.
Mr Matthews, who plans to share his winnings with the charity Macmillan Cancer Support, said the bet acted as a huge incentive for him to stay alive.
He said: "In a way it's a shame the bet is only valid for three years because the way I feel right now I'm indestructible."
A William Hill spokesman said: "We had never been asked to accept a bet of this nature before, but as Jon approached us directly and was adamant that it would give him an additional incentive to battle his illness, we offered him the bet he wanted.
"Never, in 30 years in this business, have I been so pleased to pay a winning client."
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