Tony Blair’s Attorney-General has given warning that ministers’ safeguards for detaining suspects for up to 42 days have failed to make the proposals acceptable.

Lord Goldsmith, QC, has told The Times that even with the new safeguards, he will oppose the measures if they reach the Lords.

He said: “I remain of the view that no extension is necessary or justified.”

The key amendment, and the one on which most emphasis has been placed, was the new requirement that the Home Secretary considers that there is a “grave exceptional terrorist threat”, Lord Goldsmith said.

But he added: “The hurdle to reach that opinion is not high - it would cover cases we have already had and where 42 days was not needed.

“Indeed, I understand that the Home Secretary has said as much. This amendment does not therefore really add any requirement of exceptional circumstances.”

The former Attorney’s comments came ahead of Wednesday's showdown vote with Labour MPs under intense pressure not to undermine the Government.

Lord Goldsmith said that he “regretted” that the measure was still being pressed by the Government and repeated that he would have resigned if the Government not previously withdrawn its proposals when he was its chief law officer.

“I was opposed to 90 days: contrary to what some commentators have said I never supported 90 days - as was clear inside Government and actually known outside Government too - and would have had to resign if the original 90-day proposal had come to the Lords for a vote."

Meanwhile rebel Labour MPs were urged by Amnesty International to stand firm in their opposition to the plans.

With Labour MPs under intense pressure to side with the Government, the human rights group called on them to resist succumbing to “the climate of fear that terrorists seek to breed among us”.

More than 7,160 people have signed a petition on the Number 10 website, organised by Amnesty, opposing any increase in the current 28-day limit to detention without charge.

In a letter to backbench MPs thought pivotal to the outcome of tomorrow’s vote, Amnesty’s UK director, Kate Allen, said the vote represented a watershed for human rights in the UK.

About 50 Labour MPs have expressed concern about the measures, threatening to join the Tories and Liberal Democrats in opposing the 42-day measure in the Counter-Terrorism Bill.

While the Government has tried to buy off rebels with assurances about when it would be invoked and how early MPs would be given a say, Ms Allen insisted the concessions were inadequate.

“All states have an obligation to act to protect people from terrorism,” she said.

“The perpetrators of terrorist attacks must be brought to justice.

“However, unless governments respond to the threat of international terrorism with measures that are fully grounded in respect for human rights, they risk undermining the values they seek to protect and defend.

“You have an opportunity this week to defend the values that underpin civil liberties in this country."

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