Asked six months ago what he thought the maximum detention period for terrorist suspects ought to be, Sir Ian Blair, the Metropolitan Police Commissioner, replied: “Somewhere out there between 50 and 90 days is a limit which would seem very sensible.”

Today, the top brass at Scotland Yard are pretty much resigned to the fact that not only is 90 days way off the agenda but the Government’s preferred 42 days also looks unlikely.

The police and other frontline agencies are far from happy. They firmly believe that not having the power to detain suspects for a longer period will, in the not too distant future, allow a dangerous terrorist to go free.

The counter-terror agencies are victims of their own success. The lack of support for 42 days is, in large part, a product of the successful investigation and prosecution of terrorist cases.

There is, as Sir Ken MacDonald points out today, a 92 per cent conviction rate in terror trials. It is also a fact that the number of terrorist cases under investigation, although it remains large, is falling.

There is no widely perceived need for more authoritarian legislation. So far there has been no case where the police would have liked more than the current 28-day limit and only 11 suspects have been held for more than 14 days.

However, the nature of major terrorist cases is becoming more and more complex.

The breakthrough in countering al-Qaeda’s activities in Britain was the ricin plot investigation in 2003. The evidence involved handwritten poison recipes, documents and other exhibits hidden in the Finsbury Park mosque and crude attempts at making toxins.

Five years on, a major terror case typically involves hundreds of encrypted computer files, hidden memory sticks, thousands of encoded e-mails, mobile telephones that are used and dumped and international travel using false passports. The investigation is complex and time-consuming.

Sir Ken suggests that introducing the power to question suspects after they have been charged would weaken the case for extended detention. That idea is regarded with derision by detectives who have spent long hours in Paddington Green high-security police station interviewing men schooled in replying “no comment”.

They do not want extra time to question silent suspects, but to keep them locked up while the trail of phones, computers and credit cards produces its evidential yield.

The evidence-gatherers are good at their job. The high conviction rate in terror cases includes a growing number of defendants pleading guilty when faced with the strength of the case against them.

Despite their success in thwarting a series of plots, the authorities know that they cannot stop every planned attack.

Al-Qaeda, they believe, is licking its wounds after repeated defeats in Britain and elsewhere in Europe. The nature of the beast is that it will return with more ambitious plans for mass murder.

To date, many of the attempted attacks in Britain have been amateurish. But with young Western jihadis learning how to make effective bombs in Iraq and Afghanistan, the terrorists will become more “professional”.

It is during a wave of multiple, complex terror plots or attacks that, security sources say, the power to hold suspects for up to 42 days will be necessary.

They argue that the time to legislate for emergency detention powers is now – when it can be properly considered – rather than in the frenzied atmosphere of a national emergency.

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