The government is preparing to offer Labour MPs a major concession to avoid losing a vote on terror detentions, the BBC has learned.
Ministers want to extend the limit suspects can be held without charge to 42 days, which many Labour MPs oppose.
To avoid losing a Commons vote, the government is to suggest halving the period during which police can use these extra powers.
The BBC's James Landale said this was a "significant concession".
The government is facing a tough parliamentary battle to extend the limit on holding terror suspects without charge.
The Conservatives, Lib Dems and an estimated 50 or so Labour rebels oppose moving beyond the current 28-day limit.
This could see Prime Minister Gordon Brown lose a Commons vote.
We remain in consensus mode. This is too serious to leave to partisan politics
Home Office Minister Tony McNulty
In the event of a major terrorist incident, there would be a 60-day window for police to apply for permission to hold a named suspect longer than the normal permitted maximum, according to the draft Terror Bill.
If after 60 days they have not exercised that right, it would lapse and MPs would have to decide whether to renew it.
But now the government is expected to propose cutting the time period the emergency powers can be used to 30 days.
The maximum time police could hold a suspect would be 42 days.
If officers did not use the emergency powers until the end of 30-day period, they would only be allowed to hold their suspect for a further 12 days without charge, the BBC understands.
Our correspondent said: "This is a significant concession because it curtails police powers rather than improving the scrutiny of the way they are agreed."
"We remain in consensus mode. This is too serious to leave to partisan politics," said Home Office Minister Tony McNulty.
Labour's Keith Vaz says there is "no evidence to go beyond 28 days"
Prime Minister Gordon Brown has said he would rather "do the right thing" and lose, than back down on the 42-day limit.
The government wants to secure the support of the nine Democratic Unionist Party MPs for its plans.
One possibility might be with offers of seats on the intelligence and security select committee.
Or it could be by ensuring income from the sale of surplus Army land in Northern Ireland stays locally, rather than going to the Treasury.
Mr Brown's predecessor Tony Blair suffered his first Commons defeat when he tried to extend the limit to 90 days in 2005.
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