What are the proposals?

The Counter Terrorism Bill would extend the maximum period a suspect can be held without charge from 28 to 42 days.

Detention can already be extended for an extra 30 days under the Civil Contingencies Act, but ministers want to be able to give investigators extra time without declaring a state of emergency.

What is the case for going beyond 28 days?

The Government says the growing complexity and international nature of terror plots means that at some point police will need more time to question suspects before charging them. So far around half a dozen people have been held close to the 27/28-day limit, but there have been no cases that have reached it.

Scotland Yard Commissioner Sir Ian Blair and Northern Ireland Chief Constable Sir Hugh Orde have backed the extension, but other senior officers have expressed reservations.

Director of Public Prosecutions Sir Ken MacDonald and former Attorney General Lord Goldsmith are among legal figures who have said they see no need for the change.

Former Lord Chancellor Lord Falconer is expected to oppose the plans if they reach the upper House. The joint Commons and Lords Human Rights Committee and campaign group Liberty have also been vocal critics.

How will the powers work in practice?

The Government initially included a series of safeguards to ease civil liberties concerns, and these have since been bolstered in an effort to avert rejection by MPs and peers.

A chief constable and the Director of Public Prosecutions would go to the Home Secretary to request an extension up to 42 days in the context of a “grave exceptional terrorist threat”.

A parliamentary debate and vote must then be held within a week. If the extension is endorsed, police have 30 days to use the powers. Judges would also review the detention cases at regular intervals - scrutiny which ministers claim upholds the principle of habeas corpus.

How will the vote play out?

The Conservatives and Liberal Democrats are opposing the extension, and only a few of their MPs - including ex-Home Office minister Ann Widdecombe - are likely to buck the party line.

A few months ago Labour whips expected a rebellion by up to 50 of their MPs - a similar size to the one which defeated Tony Blair over 90-day detention.

However, concessions and strong-arm Government tactics have brought many back onside.

Others are unwilling to hurt the embattled Prime Minister - although Mr Brown has stressed this is not a confidence issue.

The Labour rebels’ own estimates suggest between 25 and 35 could vote against the extension, with more abstaining.

Much will depend on the decision of the DUP’s nine MPs, who are widely thought to be negotiating a deal with ministers. Their support would make a Government victory all but certain.

Perhaps tellingly, Home Office minister Tony McNulty, who is steering the legislation, has recently been expressing “confidence” it will be passed.

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