Hoon admits that MPs are 'thinking about their positions'

As Labour MPs return to Westminster today in a fractious and demoralised mood, senior ministers will launch a drive to defuse the rebellion over the 42-day detention of terrorist suspects.

Jacqui Smith, the Home Secretary, will outline fresh concessions to the detention proposal in an effort to minimise the Labour revolt in next week's vote. Party whips will try to gauge dissent after Labour's defeat in the Crewe and Nantwich by-election.

Opinion polls have heightened fears among Labour MPs that they are heading for disaster at the next election. No swift attempt to oust Mr Brown looks likely, but more than 40 backbenchers have privately said they favour a change of leadership.

Jack Straw, the Justice Secretary, dismissed speculation over the Prime Minister's position. He told BBC1's Andrew Marr Show: "He is the best leader we could possibly have." Geoff Hoon, the Chief Whip, said Britain faced "difficult issues", such as the detention of terrorist suspects, admitting: "MPs are thinking about the future and their own positions."

Ministers must win over half of the 50 Labour backbenchers threatening to defy the whips over the 42-day detention plans. Ms Smith will appear at a meeting of the Parliamentary Labour Party and outline plans to toughen judicial and political scrutiny of counter-terrorism measures. Her concessions will include a tighter definition of the "exceptional circumstances" under which the 42-day power is invoked, cutting the period over which it is "active" and making it easier to challenge it in court. But one Whitehall source said: "The number 42 is not negotiable."

Writing in The Times, Mr Brown said: "I believe that ... allowing up to 42 days' pre-charge detention in these exceptional terrorist cases is the right way to protect national security."

Mr Straw and Mr Hoon both insisted yesterday that the Government would win the vote. However, the outcome looks very tight and could even come down to nine Democratic Unionist MPs.

Shami Chakrabarti, director of civil liberties group Liberty, said: "If the Government is so confident in its new last-minute proposals, why doesn't it publish them? Nothing that we have seen or heard does anything other than dilute the protections of existing law. The fact remains that this was an unnecessary and counter-productive fight that they picked."

The Government faces another rebellion today on planning laws. More than 60 Labour MPs have signed a motion expressing concern over moves to tighten central control over decisions on major projects such as power stations and airports.