Spreadfair says the house price confidence index will be based on client predictions of what the average UK and London house price will be in December 2010.
Spreadfair will use the Halifax house price index to check on the accuracy of the bets.
Prices for the UK are available monthly and prices for Greater London are available on a quarterly basis.
Spreadfair says the latest Halifax numbers have come as a shock to some clients.
At the beginning of March clients predicted UK house prices would decline by about 12% to about £173,000 in December 2010.
The market has now moved significantly down and the predictions suggest the market will fall by more than 15% to around £160,000 by December 2010.
Nick Sproule, head of key accounts at Spreadfair, says: “The latest Halifax figures reported a 2.5% drop in average UK house prices and our clients’ predictions suggest a steady decline will continue over the next two years.
“Current prices for December 2010 suggest a decline by as much as 15% from current levels and there is continued nervousness in the market.”
He adds: “As these prices are derived purely from client trades we believe this to be a strong indicator of market sentiment. They give us a great insight into how the ‘man in the know’ sees house prices going over the next couple of years.”
Spreadfair says Halifax has put the average price of a London house at £304,000.
Spreadfair clients are predicting a decrease of about 18% in the value of London property by December 2010.
Although London was one of the few areas to show some strength in the recent Halifax results, clients have sold the market down to around £250,000.
They are not convinced that London prices will be able to avoid the impact of the credit crunch and as a result will continue to drop over the next couple of years.