Terrorists are plotting more numerous and deadlier attacks against Britain each year, London's police chief warned lawmakers Tuesday.

Ian Blair told a committee examining changes to detention laws that his officers face a sharply rising threat from terrorism.

"The number of the conspiracies, the number of conspirators within those conspiracies and the magnitude of the ambition — in terms of destruction and loss of life — is mounting, has continued to mount year by year," Blair told the House of Commons Home Affairs committee.

Blair told legislators laws must be tightened, allowing police more time to question suspects before they must be charged.

"If you can see the epidemic moving towards you, you start to take precautions before it arrives," said Blair, who led police during the 2005 suicide attacks on London's transit network that claimed 52 lives.

The committee is examining options to extend the amount of time police have to detain and question terror suspects. Suspects now need to be charged or released within 28 days.

Prime Minister Gordon Brown has told lawmakers that a legislative program to be announced next month may include plans to extend the limit to 58 days in specific circumstances.

Brown, whose first week in office was marked by failed terror attacks in London and Glasgow, said he would also consider an option to allow police to question suspects after they have been charged, which is not permitted now under British law.

Blair acknowledged that any extension of the 28-day limit is likely to be opposed by civil liberties groups and many of Britain's nearly 2 million Muslims, who believe they are unfairly targeted by police.

He said officers should do more to explain to Muslim communities why lengthy periods of detention are necessary.

"At some stage, 28 days is not going to be sufficient, and the worst time to debate whether an extension is needed would be in the aftermath of an atrocity," Blair said.

Human rights campaign group Liberty said Blair had failed to prove the case for an extension.

"He compares terrorism to an epidemic whilst ignoring how internment provides the best conditions for that contagion to spread," said director Shami Chakrabarti.

Opposition Conservative lawmaker David Davis said "any increase needs to be based on evidence — not guesswork," claiming Blair had failed to prove the need for an extension.

Police previously have said the complexities of trawling through seized computers and following up leads in numerous countries make terrorism investigations time-consuming.

British government security officials said MI5 — the domestic spy agency — believes the number of suspects plotting attacks against Britain is around 2,000.

Around 250 loose networks are believed to be capable of mounting an attack, with a steady number of around 30 plots being monitored by agents, the officials said.

Late last year, officials had judged the number of potential plotters at 1,600, spanning across 200 networks.

New threats constantly are emerging as other plots are halted, including some since the botched summer attacks in Glasgow and London's West End, a security official said on condition of anonymity because of the sensitivity of the subject.

Brown's attempts to extend detention limits could deliver further woes after his reputation was knocked last week over his failure to call an expected autumn election.

Extending the limits handed former Prime Minister Tony Blair his first parliamentary defeat in 2005, when lawmakers rejected a bid to allow police to hold suspects without charge for 90 days. The compromise of 28 days was reached instead.