The Prime Minister has been claiming more than £15,000 a year for his constituency home in Sedgefield, County Durham, The Sunday Times reported.
MPs are entitled to claim for the running costs of their homes, taking into account such expenses as utility bills, but it is unclear why Mr Blair is claiming more than £1,000 a month. Downing Street declined to explain but said it was not being used to pay the costs of a mortgage.
There has been heightened interest recently in the size of the Blairs' mortgage debt, estimated to be in the region of £4m.
Commons rules allow MPs to claim an "additional costs allowance" for staying away from their main home, and these can also be used to claim for expenses such as council tax.
Claims can also be made for decoration and any repairs which may be "necessary" if a property is becoming dilapidated, although anything which adds value is not allowed.
Downing Street said the Prime Minister had complied with the rules and that the home was used as an office for constituency staff.