The case for allowing police to hold terrorist suspects beyond the current 14-day limit is strong. The threat to innocent Britons of suicide attacks launched by a very small minority of extremists is real; last week the nation remembered the 52 people killed in London on July 7. This new threat — and it is unlike any terrorist threat faced by this country — is asking difficult questions of the police and intelligence services. Radical Islamic cells are hard to penetrate. They have no political agenda and no interest in debate. Their aim is to assault Western values with violence and undermine democratic societies with terror, and in doing so destroy as much life, economic infrastructure and spirit as possible. The rules have indeed changed.

The legislative response by police chiefs, supported by the Prime Minister, has understandably been regarded as “bidding high”. To extend the period for which a terrorist suspect can be held without charge from 14 to 90 days is indeed a giant leap. There is, though, a forceful case to be made for a radical change. With sparse intelligence about the “enemy” and lives at stake, police chiefs have to move early to disrupt an attack they believe is being planned. This may be before they have enough evidence to lay terrorist charges. They may face high hurdles in trying to mount such charges. Establishing an uncooperative suspect’s identity can be hard. Encrypted computer files and hours of CCTV footage take days to sift. What if officers come across an “unknown unknown” in the course of an investigation, such as a new type of explosive that requires careful and lengthy analysis, slowing them down?

The public, according to our Populus poll today, are sympathetic to these difficulties. It may be that they, better than the Conservative and Liberal Democrat leaderships, appreciate the new vulnerabilities. The public would certainly take a dim view if they believed that the Opposition was exploiting the arithmetic of a finely balanced Commons. The Tories, in particular, need to reconsider. They can sound as if the terrorist threat is secondary. To most voters, this attitude reeks of tawdry opportunism.

That said, the case mounted in public for a long extension by Sir Ian Blair, the Commissioner of the Metropolitan Police, has been less than compelling. The Terrorism Bill needs three important changes. It should be High Court, not district, judges who review a case every seven days. A case requiring a suspect to be held for longer than a month without charge should be regarded as extraordinary. And the Act, if it becomes such, should be reviewed annually. What is more, the police need to be honest about their ability to pre-empt such terrorist attacks which, in truth, is limited. Sir Ian should make clear that he will use the new crime of “acts preparatory to terrorism” to charge suspects. He must also show that the police are worthy of new powers. Four men are in custody in connection with the attempted July 21 bombings. But an innocent man is also dead, the result of eight police bullets.