Speaking on GMTV, Mr Cameron said it was not enough for parliamentarians "just to meet the letter of the rules", they also had to show that their actions were reasonable.

His comments come as Conservative chairman Caroline Spelman faces further questions over the payment of a nanny from public funds. It also follows the resignation last week of the party's leader in Brussels over the issue of staff expenses.

Ms Spelman is due to meet Parliamentary Standards Commissioner John Lyon over the allegation that she broke the rules in paying nanny Tina Haynes from public funds.

The Tory chairman has insisted she has done nothing wrong, with Ms Haynes undertaking secretarial and administrative work as well as providing childcare services outside school hours.

This account was backed by a statement released by the nanny on Saturday, in which Ms Haynes said that in 1997/98 she had "two roles, one helping Mrs Spelman with childcare and another providing secretarial help to her as a MP".

But when initially interviewed by BBC2's Newsnight last week, Ms Haynes reportedly said she did no political work for the Meriden MP, but took "the odd phone call" and posted documents for her.

The controversy follows last week's resignation of the Conservative leader in the European Parliament Giles Chichester and the replacement of Den Dover as the party's chief whip in Brussels after questions were raised about their staffing expenses.

Speaking on GMTV, Mr Cameron said "What is happening, it goes back many years. For years MPs and MEPs were given allowances and expenses because their pay was being held back - that wasn't right. What we need to do now, and what I have given quite a lead on, is we have got to be more open and accountable.

"So, of course, we have to declare what MPs and MEPs are paid but we should also declare the allowances and expenses."