The BBC's security correspondent Frank Gardner looks at what is going on.

Did the Home Office mean to release this warning?

They probably should not have released the first one - it was not yet authorised and had not been approved for release. It was an early draft and I think they probably scared people more than they wanted to by that.

But they are trying to get people to be more vigilant.

The intelligence services have picked up so many warnings signs that they felt that they had to say something to the public.

The Americans and the Germans are doing the same. There's a group called Al-Tahid who the Germans believe are responsible for the attack on a synagogue in Tunisia in April this year and they believe there is a very high risk that Al-Tahid, who are linked to Al-Qaeda, will try to do something similar in the next three weeks.

In the initial announcement there was specific reference to a dirty bomb or some kind of poison gas, what is a dirty bomb?

It is basically a radiological bomb made from conventional explosives with radioactive elements wrapped around it - the kind of thing found in hospital x-rays.

If it explodes it scatters radioactive debris which doesn't kill a lot of people but can make quite a large area uninhabitable for a long time. If you spend a lot of time handling the debris you can die but it's really a weapon of mass disruption rather than destruction.

So from the warning do we know that Al-Qaeda are alive and well and potentially pose a daily threat in this country?

Potentially. The government is taking a lot of precautions now. They have started to issue chemical protection suits for hospitals, there are emergency teams on standby and they have started to stockpile extra medicines. They are very alert to the possibility of biological terrorism.

The US has done the same - they have invested $6bn (�4bn) in trying to defend against biological terrorism because the US is still the big target for Al-Qaeda.

A lot of people are wondering 'why haven't we been hit here in Britain?' because we are such a close ally of the United States. The truth is that any attack by Al-Qaeda or its affiliates on Britain will hurt Muslims as well.