Yesterday Downing Street refused to explain where the money went, but admitted for the first time that it was not being used to pay the costs of a mortgage on the property.

MPs are allowed to claim running costs, such as utility bills, but a spokeswoman declined to explain how Blair claims more than £1,000 a month for costs other than a mortgage.

Blair, who bought the house in 1983 for about £30,000, claimed expenses of £8,001 for it in 2001-2. But in 2002-3, his claim rose to £19,538. In each of the following two years he claimed more than £15,400.

The national average cost of running a home is estimated at £8,530 a year — and that includes mortgage interest. According to Land Registry documents, Blair “discharged” a loan he used to buy the home in Sedgefield when he took out a new mortgage on the property in December 2003 for at least £200,000. The new loan means the Blairs’ total mortgage debt — including £3.47m on a London home — is more than £4m.

Downing Street said the increase in his expenses claims was not connected to the remortgage on the constituency home. A spokeswoman said: “He did not claim any allowances to pay for interest on the additional loan. The allowances would have been used in the normal way, but not on the additional remortgage amount. He has complied with the rules.”

Commons regulations allow MPs to claim an “additional costs allowance” (ACA) for staying overnight away from their main home.

The ACA can be used to pay interest on a mortgage to buy a property, but not to pay interest on any subsequent loan. The rules also allow MPs to claim for heat, light, water, insurance and council tax. Average utility bills in the area of Blair’s constituency home come to about £950 a year. The house, called Myrobella, falls in council tax band D, which cost £1,293 in 2004-5. Insurance costs and phone bills are unlikely to exceed £1,500 a year. In the past four years, about £45,000 of Blair’s expenses are not publicly accounted for.

The ACA rules allow MPs to claim for decoration and “necessary” repairs to make good dilapidation. But improvements that increase the value of a property cannot be claimed.

Downing Street said Myrobella was also used as an office for constituency staff, but refused to detail whether that affected Blair’s housing allowances.

The house has undergone extensive security upgrades, largely paid for out of central government funds. Downing Street would not comment last week on whether any of the expenses were security-related.

Although there is no suggestion Blair has broken any rules, the apparent “black hole” in his expenses raises new questions about ministers’ use of parliamentary allowances, which critics say are open to abuse.

This weekend, Sir Alistair Graham, chairman of the committee on standards in public life, said there should be regular reviews of the expenses to ensure they were in line with those of other public bodies.

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