Frank Gardner BBC security correspondent

Government sources say this was an intelligence-led operation and that it was done through tip-offs.

It was a joint operation with the security service and the Metropolitan Police and of course, it has got major health issues attached.

This is the most significant find of poisonous materials in Britain since 11 September 2001 in relation to terrorism and possible attacks on Britain.

The subject found - ricin - is more deadly than cobra venom or cyanide.

The six men who are under arrest are of north African origin, mostly from Algeria.

Of course, one line of inquiry for the police will be of any possible links with al-Qaeda, although investigations are continuing.

Ricin is a very toxic poison which was used some years ago by Bulgarian secret agents to assassinate a dissident in London by injecting a pellet.

It is a contact poison which means that if you eat it or breathe it you'll be affected.

It is not contagious like a virus.

With the quantities of ricin found it is not thought that it was necessarily a chemical bomb that was going to be detonated.

But still, it is an extremely serious poison and of course it is raising all sorts of questions about what people planned to do with this.

Psychological damage

Possibilities are being looked at such as was it going to be smeared onto doors, onto some area where lots of people would touch it?

Al-Qaeda and its affiliates have made a strategic decision in the last few months that Britain is a legitimate target for them

It would not, in this form, necessarily kill a lot of people, but the psychological damage would be enormous.

This is the most significant find of poisonous materials in Britain since 11 September 2001 in relation to terrorism and possible attacks on Britain.

So it is very important, even though the actual volume was relatively low this is a very significant find.

Serious chemicals

This was what would be called a 'specific threat'.

For example, when we had the London Underground scare there was no actual specific threat.

In this case there really was - these are serious chemicals.

Al-Qaeda and its affiliates have made a strategic decision in the last few months that Britain is a legitimate target for them.

Initially they wanted to target America and Israel.

They have broadened that to Britain, France, Italy, Spain and Australia - countries which they see as being part of the US-led coalition in the War on Terror that is targeting Muslims.

When the British government or various agencies have issued these often rather garbled, confusing and slightly hyped-up warnings about non-specific threats they are in a way covering themselves.

They see so much intelligence, some of it true, some of it false about possible attacks on Britain and Europeans using poisonous chemicals they feel they then have to make a warning.