Downing Street is being forced to rethink plans to fight for the bitterly controversial move amid fears that the issue could be turned into a vote of no confidence in the wake of last week's humiliating by-election loss in Crewe and Nantwich.

A phone round of MPs will be made by No 10 this week to establish the level of opposition in the party. If the vote cannot be won, sources say Brown will back down and accept the current limit of 28 days, despite his claim last month that he was going to try to "win the argument" with MPs, no matter the result.

Brown had been hoping to win over up to 50 Labour rebels with concessions, which would have meant that the 42-day limit would be used only on rare occasions, such as when the authorities discover evidence of multiple terrorist plots.

However, several Labour MPs opposed to the bill signalled last night that they would not be persuaded, warning they would even ignore a plea to remain loyal to an embattled Prime Minister.

The tacit acknowledgement of his weakened position comes after the worst few days of the premier's spell in Downing Street and amid fevered speculation among Labour MPs and ministers over whether he should quit.

One minister told Scotland on Sunday that MPs would not simply accept that defeat to the Conservatives was inevitable at the next general election. The source said Cabinet ministers were being made "aware" that one possible route would be for a delegation to simply tell Brown that his time was up, and that he should go.

However, a millionaire Labour donor who helped bankroll Brown's uncontested leadership campaign said the Prime Minister needed to be "much tougher".

Lord Paul said he was "depressed" at Labour's present woes, warned there was little time to get things back on track, and called for a Cabinet reshuffle.

The businessman gave £45,000 to the Brown camp and went on to pledge "whatever I can pay" last autumn if the Prime Minister went ahead with a snap general election.

The peer said: "I don't think we have a better man than him at the moment, but he has to exert his authority. I think he's too gentle.

"If you ask me, I feel he should really have been much tougher from the beginning. Gordon has to exert his authority further."

And several leading party figures yesterday rallied round Brown, with former deputy prime minister John Prescott urging MPs to "get behind" the Prime Minister. One source said all attention was on avoiding another disastrous defeat over the terror Bill.

One insider said: "There is going to be a phone round this week of MPs. To bring on a revolt would lead to even more speculation that he hasn't got the authority to lead any more."

Midlothian MP David Hamilton said last night his mind would not be changed.

"There are certain issues that you don't back down on. Gordon might try and issue the loyalty line, but on the question of 42 days you are not going to convince people like me. It is a bigger issue than the party."

However, Hamilton urged MPs to get behind Brown. "He has got to re-establish that he is the strongest person to lead the party forward and he can do that," he said.

Shami Chakrabarti, director of pressure group Liberty, has this weekend written to Labour MPs urging them not to back Brown's concessions.

She declares: "The clearest option would be for the proposal for 42-day detention to be dropped altogether, acknowledging that the case for any extension beyond 28 days has not been made out."