MPs will be given a vote on whether to trigger emergency powers that would allow terrorist suspects to be held beyond 28 days, under plans to be announced by the Home Secretary within days.

The move to break the deadlock between the parties over detention will hand MPs and peers the final say on police requests to continue questioning suspects. It would introduce an unprecedented element of parliamentary control over and debate about the conduct of fast-moving security operations.

Gordon Brown is pressing Labour MPs to back the package shortly to be announced by Jacqui Smith. It follows lengthy cross-party talks aimed at reaching a consensus on the most difficult issue now facing Parliament. The plan is intended to be a “model that will combine judicial and parliamentary scrutiny”, a senior Whitehall figure told The Times yesterday.

Ministers are expected to duck the issue of the maximum period suspects can be held without charge, however.

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A Home Office spokesman said: “We have been clear that we believe that there is a case for going beyond 28 days in future and have consulted widely on options for how this might work in exceptional circumstances.”

Ministers are confident they can wrongfoot opposition parties by accepting their demands to give Parliament greater scrutiny over the detention of terrorist subjects.

In particular, Ms Smith plans to use comments by David Davis, the Shadow Home Secretary, endorsing emergency powers in the Civil Contingencies Act to try to win cross-party consensus on the issue. The Act allows for suspects to be held for a 30-day period after the current 28 days. The Tories have already accepted the principle that Parliament can – in exceptional cases – extend detention times, she is expected to claim.

The proposals are intended to cover circumstances short of a full state of emergency, for example after the discovery of multiple, complex plots. The option that the Government is now favouring appears to be one that it described in its initial consultation document as impractical and a cause of operational difficulties for police.

It envisages a vote in Parliament during a major antiterrorist operation – an idea that alarms senior police officers and prosecutors because of the threat that sensitive evidential matters might be discussed, vital time lost and an investigation undermined.

There are further concerns that a heated parliamentary debate about a live investigation, followed by a vote in both Houses, would run the risk of prejudicing a future prosecution.

Defence lawyers would be able to argue that publicly debating the need to detain their clients was prejudicial and, potentially, a contempt of court.

However, Ms Smith and Mr Brown have had to give ground in the face of pressure, not least from within his own administration. Baroness Scotland of Asthal, QC, the Attorney-General, Vera Baird, the Solicitor-General, and Ken Macdonald, the Director of Public Prosecutions, are all believed to be sceptical of the need for a detention limit beyond 28 days.

Sir Ian Blair, the Metropolitan Police Commissioner, has said that a detention period of 50 to 90 days would be “sensible” if there were judicial oversight, however.

Lord Carlile of Berriew, the govern-ment-appointed reviewer of antiterrorism laws, has said that there are “going to be a few cases in which more than 28 days is going to be needed for proper investigations to be carried out”.

Sir Ian told the Commons Home Affairs Select Committee in October that Parliament needed to resolve the detention issue now to avoid having to debate it during an emergency. He said: “At some stage 28 days is not going to be sufficient, and the worst time to debate whether an extension is needed would be in the aftermath of an atrocity.”

Sir Ian added: “The number of the conspiracies, the number of conspirators within those conspiracies and the magnitude of the ambition, in terms of destruction and loss of life, is mounting, has continued to mount, is increasing year by year . . .

“If you can see the epidemic moving towards you, you take precautions before it arrives.”

Scotland Yard has declined to specify a number of days but Sir Ian said he believed the power required was the ability to detain suspects for between 50 and 90 days.

Sir Ian said he regarded lengthy detention without charge as an affront to the normal rule of law, but it was a power that was necessary and one that would be used “immensely sparingly”.

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