By Nick Assinder BBC News Online political correspondent

How and when to warn the public of a heightened terrorist threat is always going to be a hugely difficult call.

Overdo it - as the Home Office was accused of doing last week - and you can spread panic and hand the terrorists a victory by paralysing normal, everyday life.

Too much caution, on the other hand, simply adds to a more general air of apathy - the "we've heard it all before" tendency.

Blair's impossible choice

And, unlike the US where domestic terrorism is a relatively new thing, Britons are particularly hardened to a persistently high level of threat.

But when the prime minister uses one of his few set piece speeches of the year to warn the public to be extra vigilant, then it's probably a good idea to listen.

High threat

And in the wake of the Bali bomb and the claims and counter claims over who knew what and when, few will complain if Tony Blair is erring on the side of caution.

The truth, as far as we are being told it, is that intelligence has revealed significantly increased activity by al-Qaeda which could point to an attack.

That is not to say, however, that a specific attack or threat has been identified.

But Downing Street is stressing that the threat is as high as it was this time last year, immediately after the 11 September atrocity. The government is clearly very worried.

Spokesmen are also eager to point out that the nature of the threat is vastly different from the one posed by the IRA - it could be either conventional or unconventional and the possibility of suicide bombers cannot be underestimated.

Eyes open

And that makes it hugely difficult for members of the public to know exactly what they are looking for.

But, if the intelligence is there, most would far rather the government issues such a general "keep your eyes and ears open" warning than keep quiet for fear of spreading panic.

Part of Mr Blair's message is also clearly designed to prepare the public, particularly in the pre-Christmas rush, for an increase in the number of armed police patrolling airports, stations and the streets.

This all revives memories of the worst days of the IRA's campaign in the 1970s but with added complications.

It is going to be another tense Christmas.