A couple struggling to sell their acclaimed home in Cheltenham are bidding to beat the housing slump by offering the property as first prize in a competition raffle - along with its contents.

Tim and Zoe Bawtree built the £845,000 "underground house" in their back garden for TV's Grand Designs show, but after failing to sell it through the usual channels they dreamt up the contest.

The couple are inviting would-be property tycoons to enter a prize draw at £25 a ticket, with the winner getting the keys to 12 Oxford Walk, half of which is beneath ground level.

Everyone who enters must name the horse which won the Cheltenham Gold Cup this year to stand a chance of claiming the property.

The three-bedroom "eco-home" will only be "given away" if 40,000 people enter to raise the asking price, which rises to £1m, when stamp duty, Pay Pal fees, furnishings and a donation to Cancer Research are factored in.

They are in negotiations with a celebrity to make the new year draw and it will be announced to the world on TV and via a webcast.

Mr Bawtree, 37, the director of a software company with a £500,000 annual turnover, said: "We put the house on the market for three months and while we had interest from potential buyers it did not sell. Times are tough with a slump in the market so we looked into how we could do it differently.

"We spent a lot of time finding a solicitor and talking to the Gambling Commission. Some of our friends and family thought we were crazy when we told them, but whereas with a lesser-known house people might think it was a scam, this property is very well recognised, because of Grand Designs.

"Someone entering this could find themselves with a mortgage-free step onto the property ladder or a luxury second home.

"The rental value is excellent at £2,500 a month and it's a win-win situation because the charity should get £110,000. Like many people, we've lost family and friends to cancer."