A desperate Mr Brown, who saw his majority of 65 wiped out by furious Labour backbenchers, had last-minute secret meetings with the DUP's nine MPs before they decided to vote with him.

When the result was announced to a packed House of Commons, Conservative MPs, including leader David Cameron, turned round to point the finger at the DUP, some shouting "traitors" and "you've been bought".

But the Prime Minister and the DUP yesterday continued to insist they had taken a "principled" decision to back the Government's controversial plan to lock up suspected terrorists for 42 days without charging them with any offence.

The DUP found itself in the rare position of being central to the outcome of such a crucial vote and at 4pm on Wednesday its press office released a statement from Nigel Dodds claiming the party had not yet decided which way it would vote.

But following a private meeting with the Prime Minister an hour later, the nine MPs followed the Government into the "yes" lobby to cast their votes.

Mr Brown rejected claims that he had bought their backing, or that of some Labour MPs, with promises of action on issues such as extra cash for Northern Ireland or a peerage for Ian Paisley.

Speaking at his Downing Street Press conference, Mr Brown said: "There was no deal. There was no deal with the Democratic Unionist Party, no deal with the Ulster unionists generally.

"And I think for people to imply that is to take away from the strength of the argument about the need to tackle terrorism.

"Nobody knows more about the need to take action in preparation in case you have terrorist abuses than the people of Northern Ireland.

"And I believe that the people who voted for this voted on principle because they are persuaded by the argument."

Suggesting the DUP backed the Government only because of offered incentives was "impugning the reputation of people who are concerned about terrorism", he told reporters.

Westminster has been rife with rumours of deals offered to wavering Labour MPs to secure their support.

But Mr Brown said: "I do not offer deals on terrorism."

And South Antrim DUP MP William McCrea said: "I want to make it clear that we voted for this legislation because we believe in strong anti-terrorism measures to protect the safety and security of the people of the United Kingdom."

But Traditional Unionist Voice MEP Jim Allister criticised his former party.

"What a farcical irony in the DUP MPs voting for 42-day detention for suspected terrorists, when every day the DUP sustains in government convicted terrorists, some of whom deserve 42 years," he said.

And Alliance leader David Ford described the DUP's decision as an "outrage".

"They opposed internment in Northern Ireland but have now effectively supported imprisonment without trial in Great Britain," he said.

"I don't know what reward they got from Gordon Brown but in my opinion liberty is priceless and should never be sold."