Gordon Brown and Jacqui Smith have been allowed a further two months to persuade Labour MPs to support plans to allow terrorist suspects to be detained without charge for 42 days.

The long-awaited Counter Terrorism Bill is to have its second reading in the Commons on April 1, the Government confirmed yesterday.

But the Conservatives have decided against trying to kill the Bill at its earliest stage, which would have been possible by tabling a 42-day amendment for the second reading debate.

Shadow ministers have decided to try to remove the 42-day clause when the Bill returns to the floor of the Commons, probably in late May, after its detailed committee stage.

That means the 42-day decision – which ministers say represents Mr Brown’s greatest risk of a serious defeat since he became Prime Minister – will not be taken until after the May local and London elections. Ministers and whips insist they are making good progress in trying to convince worried Labour MPs that the provision would be used only in exceptional circumstances and after substantial parliamentary and judicial oversight.

“Once they understand this is not a blanket provision which would allow anyone to be held for 42 days, the opposition begins to subside,” a senior MP said last night. The current detention period is 28 days.

In theory, the Opposition could have tabled a “reasoned amendment” declining to give a second reading to the Bill because of the 42-day provision. Privately, ministers had hoped that might happen so that they could accuse the Conservatives of being “soft” on the terror Bill. But Conservative sources said last night that there was much that they could support in the Bill and that they would concentrate their fire against the 42-day plan at report stage.

Times Recommends

• 5 Christmas romance confessions
• Has champagne lost its sparkle?
• Your tailor-made Christmas