Pressure on Smith over her £116,000 claims for family home

The Tory leader said Ms Smith may have some "questions" to answer over her arrangements for claiming £116,000 over six years.

The Home Secretary has been engulfed in controversy after it emerged she has declared a house in London that she shares with her sister as her main residence. This allows her to claim up to £24,000 a year for a second home in Worcestershire where her husband and children live.

Ms Smith insists that she has not broken any Commons rules. But she could face an investigation by the parliamentary watchdog after a campaign group reportedly made a complaint about her use of the funds.

Mr Cameron questioned the conduct of the Home Secretary, whose post comes with a £142,000 salary. He said: "She may have some questions to answer when you look at the Green Book and all the rules that govern it. There are rules and you have to meet both the letter and the spirit and explain in a reasonable way this is the arrangement I have and I think it's a reasonable one'."

MPs in the Cabinet have claimed more than £2 million over the past six years for second homes and they are split over which property they treat as their main residence. Children's Secretary Ed Balls and his wife Yvette Cooper treat their Yorkshire constituency home as their main residence, allowing them to use taxpayers' money to pay for a £600,000 second home in the capital.

Culture Secretary Andy Burnham, Communities Secretary Hazel Blears and Universities Secretary John Denham also declare their constituency home as their main home.

However, Justice Secretary Jack Straw and Health Secretary Alan Johnson have their main home in London, with the latter stressing that he rents a "modest" flat in Hull.

There is no evidence that any ministers have broken any rules. Mr Balls and Ms Cooper were cleared of any wrongdoing by Parliament's sleaze watchdog. But the Commons guidance is widely seen to be so lax that MPs can effectively choose which property to declare as their main residence. Under the rules, the main residence is where an MP "spends more nights than any other".

Ms Smith says she has fully abided by the Commons regulations and that her arrangements, under which she spends the majority of her time on government business in London, had been approved by the parliamentary authorities. She usually returns to her constituency home on Thursday evenings and is back in London on Sunday or Monday. Ms Smith is said to pay her sister Sara, a BBC reporter, a "market rate" to live in their shared house.

A spokeswoman for Ms Smith said: "She spends the majority of her time in London and has full approval for any associated expenses relating to her second home in her constituency."