The disclosure followed a powerful call by the Tory leader for the 'full force of the law' to be deployed against MPs who have abused allowances.
Following a Mail on Sunday investigation Mr Cameron could now face searching questions about his own expense claims.
He took out the £350,000 mortgage - close to the maximum amount that can be claimed for - to buy a large house in Oxfordshire in August 2001, two months after winning his Witney seat in the General Election.
By nominating it as his second home, he was able to claim for the mortgage interest payments under the now-infamous Commons' Additional Costs Allowance (ACA).
There is no suggestion that he broke any rules. But mortgage experts say that if he had kept the loan on his London home and borrowed £75,000 less on the Oxfordshire property, taxpayers could have been saved more than £22,000 between 2002 and 2007.