Parliament's expenses watchdog has insisted it would not step back from its duty to protect taxpayers' money, after David Cameron backed MPs' complaints about the tough new regime it has introduced.

While expenses rules should be "workable and fair to MPs", the much-criticised Independent Parliamentary Standards Authority (Ipsa) said its "overriding responsibility" was not to MPs but to the taxpayers who pay for the system.

Since it took over paying expenses in April, Ipsa has been accused of imposing administrative burdens on MPs and stopping them doing their jobs properly.

The Prime Minister stoked the controversy by branding its rules "anti-family" and warning that it must improve by April or he would force changes.

Mr Cameron's spokesman declined to rule out the option of scrapping the new body, but told reporters: "I don't think we are there yet, I think we need to look at what can be done."

The Government was not setting out a position on how the system should be reformed, which was primarily a matter for Parliament, said the spokesman.

Complaints about the excessive bureaucracy of the new rules on expenses have been voiced by MPs from all parties, including Tory backbenchers who raised the issue with Mr Cameron at a meeting of the 1922 Committee. The PM told them he "recognised that (Ipsa) has caused a lot of pain and difficulty", and criticised the new arrangements. "It is anti-family and it is not acceptable," he said.

According to aides, Mr Cameron said a "better system" needs to be in place by April 1, otherwise it "would be changed".

In a statement, Ipsa said: "When we made the new rules, we gave the public a say in setting them - for the first time ever. And it is worth remembering that all the main parties and political leaders welcomed the rules and the changes we introduced.

"We have a duty to administer a system which is workable and fair to MPs. But we also have an overriding responsibility to the public. We take very seriously the task of trying to restore public confidence in how MPs are supported by the taxpayer. We will not step back from our duties to the public."

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