The power to invoke the 42-day limit for detaining terror suspects could be triggered by events well short of a genuine emergency, opponents insisted last night.

The chairman of the Joint Committee on Human Rights sought to demolish the Government’s concessions in a letter to the Home Secretary on the eve of the crucial vote.

Andrew Dismore, the Labour MP for Hendon, told Jacqui Smith that the newly introduced definition of a “grave exceptional terrorist threat” under which the power would be used was “extraordinarily broad”.

He said: “[It] includes events or situations which fall well short of constituting a genuine emergency in any meaningful sense of that word.”

The second concession, that Parliament would have to approve the measure within a week, does not offer a serious safeguard since MPs would be limited in what they could say about any case, added Mr Dismore. “There is a risk that the parliamentary debate will not only prejudice possible future trials, but will prejudge some of the very issues which a court will have to decide very shortly afterwards.”

Ministers were also accused last night of “sexing up” the case for extending to 42 days the period that terror suspects can be held without charge. Supporters of the measure say that key evidence came to light only at the end of the current four-week maximum in three cases presently before the courts. But Shami Chakrabarti, the director of Liberty, said that she had spoken to a lawyer working on two of the cases who insisted that the key evidence came to light on the fourth and twelfth days of the suspects’ detention.

“I am shocked to find ministers have repeatedly sexed up the operational pressures on the existing 28-day detention limit. I hope that the similarities with the infamous Iraq vote will not be lost on Labour MPs,” she said.

Muhammad Abdul Bari, the secretary-general of the Muslim Council of Britain, urged MPs to resist the plans. He said: “Of course it is right that we take proper precautions against the threat of terrorism. However, it is our view that this legislation will be counter-productive and will play into the hands of extremist groups.”

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