So Prime Minister Gordon Brown has succeeded – by the skin of his teeth — in getting Britain’s House of Commons to approve new police counter-terrorism powers that were condemned by civil liberties groups, a former prime minister, a U.N. human rights investigator and several dozen of Brown’s own Labour MPs. The Guardian newspaper writes about ‘Liberty, security and an anxiety over lost rights’.

And even the government admits the power to hold terrorism suspects for up to 42 days before charging or releasing them has never been needed until now: it wants it as an insurance policy against future attacks or plots in which the police may need more than the 28 days they now have in order to investigate tangled international links, false identities and masses of encrypted computer files.

So what’s going on? The bald figures suggest Britain is way out of step with other democracies. The six weeks allowed under the bill for initial questioning of terrorism suspects compares with one day in Canada, two in the United States, Germany, South Africa and New Zealand, five in Spain and 12 in Australia.

But the bald figures don’t tell the whole story. Police in most European countries, for example, hand cases over to a judge or prosecutor after the first few days and the suspect may wait in jail for months or years while the investigation proceeds. Britain can also plausibly argue, on the basis of the number of plots intercepted in the past few years, that it is more threatened than most countries by al Qaeda-inspired militants.

Opinion polls suggest the public backs Brown on this issue, although his overall popularity rating is dire. And with the House of Lords likely to oppose the bill and send it back for re-consideration by the lower chamber, Brown is far from being out of the woods.

Expect more debate in coming months on possible alternative means of tackling terrorism — particularly on whether to let British police, like their counterparts nearly everywhere else, use evidence from tapping suspects’ phones as ammunition to prosecute them in court.

Despite the embarrassment caused this week when a senior security official left top-secret intelligence documents on a train, the British authorities have a strong record in countering terrorism. Since 2004 the country has seen at least one major plot each year, and many smaller ones. Only one succeeded: the July 2005 London suicide attacks that killed 52 people. So far, 2008 has been a quieter year — but the emergence of any major new threat could once again shift the goalposts in the security debate.