Many are believed to be deeply unhappy at the proposal but rebels are not expected to force a vote at this stage on the Counter Terrorism Bill.

Prime Minister Gordon Brown may face a media grilling over the controversial measure after a human rights watchdog warned it could launch a legal challenge.

Mr Brown will face the media for his regular Downing Street press conference just hours before the Second Reading debate begins on the package of anti-terror measures.

Home Secretary Jacqui Smith said at the weekend that she had been receiving a "good response" in discussions with Labour MPs about the Bill.

Mr Brown's predecessor Tony Blair suffered his first Commons defeat when MPs threw out his proposal for a 90-day limit in favour of a 28-day compromise.

The Equalities and Human Rights Commission said that it believed the new plans to extend the limit were discriminatory and in breach of the Magna Carta.

Liberal Democrat leader Nick Clegg indicated that though his party would fight "tooth and nail" to stop the extension, it would not oppose the overall package at this stage.

"We will seek to amend the Bill in committee before we agree to sacrifice the good to get rid of the ugly," Mr Clegg said.

Battle lines will be drawn when the Commons goes on to discuss the Bill in more detail, which is not expected to happen until after May 1's local elections.