Anti-terrorist police have come close in the cases of six suspects to needing more than 28 days to detain them, the former head of Scotland Yard said yesterday.

Lord Stevens of Kirkwhelpington, the former Commissioner of the Metropolitan Police who is an adviser to both Gordon Brown and David Cameron, said that the police needed to have the power “in their back pocket” to hold someone for up to 42 days in “exceedingly exceptional circumstances”.

“It is the duty of those of us who have been on the front line – losing sleep, working through the night on these cases to thwart attacks – to put forward the argument for this extra power,” said Lord Stevens, who has also investigated police corruption in Northern Ireland.

“We have achieved considerable success against al-Qaeda, but we cannot take our eye off the ball. The dangers that are out there now are not going away. In a very few cases, and I suspect it will be a small number, the police service will need this power.”

Lord Stevens, who left the Met in 2005, was angry that the security case for extended detention had been undermined by the “outrageous abuse” of surveillance powers to spy on litter louts, benefit cheats and petty offenders. “For local councils to be using such powers brings the whole security issue into disrepute,” he said.

He also echoed the fears of police colleagues that government amendments had made the procedure for triggering extended detention convoluted. Lord Stevens said: “When you’re dealing with a complex operation, deploying hundreds of surveillance officers and making delicate operational judgments, the last thing you need is a procedure that is too complex. My plea is for Parliament to make the system as simple as possible.”

With Scotland Yard and MI5 cautious about becoming involved in a rancorous political argument, Lord Stevens’s last-minute intervention in the 42-day debate is highly significant. In a police career spanning four decades he had direct experience of tackling the IRA and al-Qaeda but also confronted abuses perpetrated by some of the security agencies in Northern Ireland. He said: “I have seen at first hand the results of people abusing the powers available to them. Perhaps more than anyone in this debate I understand the fear that people have about the abuse of power.”

Lord Stevens did not speak in support of the 90-day detention proposal, which was defeated in 2005, but said that there was now a powerful case for going beyond the current 28-day limit. “I don’t look at it as 42 days and I would not support an automatic jump from 28 to 42 days. But there will be really exceptional cases where the Police Service will need to go beyond 28 days.”

Lord Stevens was at Scotland Yard when Dhiren Barot and seven other men were arrested in August 2004 after the discovery of a plot to bomb buildings in London and New York. In the first days of the inquiry, code-named Operation Rhyme, police seized hundreds of computers, hard drives, memory sticks and discs that had to be examined within the 14 days that suspects could then be held for.

He said: “We sent the encrypted material to every possible agency – from GCHQ onwards – and they struggled to get into it. By the twelfth day we had accessed less than 10 per cent of the computers but we just, only just, got enough to charge Barot within the time. It was on the edge.” All eight men later pleaded guilty or were found guilty and the investigation proved the trigger for extending detention without charge from 14 to 28 days.

Lord Stevens said: “The law at that time was not sufficient to deal with the complexity of the case. But things have moved on since my watch. The technology is more advanced, the investigations are more complex and we are at the stage where we need to legislate. It is not good to look back in the business of terrorism. It is the duty of government, police and the security services to look to the threats of the future.”

The former commissioner said that he strongly disagreed with Sir John Major, who wrote in The Times last week that the 42-day power was unnecessary in dealing with a terror threat no more potent than that posed by the IRA. “The aim of al-Qaeda is to inflict mass casualties, they don’t mind how many people they murder and maim, they don’t mind who the victims are,” Lord Stevens said.

“The IRA killed people but it did not operate on the scale of mass destruction that is al-Qaeda’s objective. The threat is vastly different. I’ve got a lot of respect for Sir John Major, but I just wonder if he was slightly missing the point in terms of the threat we face.”

Lord Stevens also decried the descent into party politics of a debate that should have been about security. He added: “As a crossbencher I will be making a plea that this issue does not become further politicised.”

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