BBC home affairs correspondent Margaret Gilmore analyses the overall terrorist risk to the UK.

Would it be possible to stage a gas attack on the Tube?

It is certainly a scenario that the security forces and the police are taking extremely seriously.

We know that the number of intelligence reports intercepted since the summer has increased massively, suggesting that al-Qaeda in particular, but other groups too, want to target the UK.

The number of intelligence reports intercepted since the summer has increased massively

We know that Bin Laden has said this himself, in the tape last week which is widely believed to be genuine.

And they do not know where this attack could come - it could be the Tube, it could be anywhere else.

We also know that, unlike the IRA which would give a warning, al-Qaeda will use suicide bombers - and instead of giving a warning will go in and want to cause mass casualties.

These warnings are being taken extremely seriously, but there is no specific target as yet coming through in these increased intelligence reports.

How well prepared are we?

We are better prepared than we were before 11 September. The nature of the threat has changed, so we have had to change the way we look at things.

A third to a half of all hospitals are not yet fully prepared for dealing with a biological or radioactive attack

No longer are we talking a small number of casualties - there is the potential for very large numbers of casualties.

And we know that about a third to a half of all hospitals are not yet fully prepared for dealing with a biological or radioactive attack.

They are moving that way, but the government has admitted there is still a long way to go.

What concrete responses are being prepared?

We are seeing far more of these really big emergency planning operations, where they go through the rehearsal and bring everybody in.

We are seeing bunkers being set up in the major cities

We are seeing new committees being set up, we are seeing bunkers being set up in the major cities.

So we are moving forward, but we are not there yet.

The ambulance brigade in London for example could cope with say 500 casualties, but certainly not in the numbers we saw at the World Trade Center.