William Hague has accused the prime minister of "extraordinary arrogance" in bringing Peter Mandelson back to government only 10 months after he quit in disgrace.

The Conservative leader promised his party would work with the new Northern Ireland secretary, but insisted it had the right to question the appointment

"If Roland Rat was appointed Northern Ireland secretary I would say everybody should work with him, but I would still point out that he was a rat."

Peter Mandelson takes office with Mo Mowlam's backing

Mr Hague said he believed it was wrong in principle to give a senior government post to a former minister who had been forced to resign less than a year before.

"He resigned in disgrace, he resigned for good reasons and if it was right for him to resign then I think it is wrong to bring him back now without him having faced election," he said.

"In the Conservative government when someone resigned in difficult circumstances, they were only ever brought back once they had faced the electorate and that will be the case in future Conservative governments.

"I think integrity and a record of truthfulness should be factors in holding government office."

But Mr Mandelson countered by claiming that his Conservative opponents were trying to "destabilise" him before he had got started in his new job.

He suggested his resignation when the details of his �373,000 home loan from a fellow minister became known set a higher standard for public life than had previously been followed.

"I did pay a very heavy price for it. I am absolutely clear it was the right thing to do at the time. I feel in a sense chastened by it," he said.

Some see Peter Mandelson in a less positive light

"But during the interval I'd used this time to put my affairs in order and I think that having put everything right and this behind me, the prime minister decide that it was right to put the events of 10 months ago behind us and give me the chance to do another challenging job."

The Standards and Privileges Committee of the House of Commons had concluded his actions in seeking a mortgage for his Notting Hill house had not been dishonest, the Northern Ireland secretary said.

But the Tory leader said questions remained over areas such as tax liability.

"And I think the whole atmosphere and questions that arose at the time of his resignation have not been satisfactorily answered.

"A minister who resigns in those circumstances by common consent and agreement should not be returned 10 months later.

"It shows extraordinary arrogance on the part of the prime minister."

David Trimble: Unionists welcomed the appointment

Mr Hague called on the Northern Ireland secretary to assert his authority by declaring a halt to paramilitary prisoner releases for organisations not starting to disarm.

"It is undoubtedly time for the paramilitary organisations to really play their part in the agreement," he said.

But Mr Mandelson had words of caution for those who hope he can clinch a breakthrough in the peace process because he is more highly regarded by unionists than his predecessor Mo Mowlam.

He said: "There is nothing that I can do in Northern Ireland except build on the basis of what Mo Mowlam has already achieved.

"I have to do exactly as she did - keep working to show a determination and bring people together to enable people to understand not only that there is a way of living together but to do so on the basis of trust.

"That is what I am going to do, to spare no effort, no ounce of energy that I have inside me to make that agreement stick."