Experts say the discovery shows the threat of terrorist activity in the UK cannot be ignored.

Professor Paul Wilkinson, a terrorism expert from St Andrew's University, said the find had not taken him completely by surprise.

Speaking on Sky News, he said: "We are facing an extremely dangerous form of terrorism.

"This particular incident seems to reveal that we should have the maximum proportions in place and preventative mechanisms against the kind of chemical, biological and possibly radiological weapons that terrorists are undoubtedly able to get these days.

"It doesn't totally surprise one because, of course, one of the scenarios that the police and intelligence services have been investigating is the possibility of a poison attack of some kind using a toxin or a chemical weapon."

It is correct to be realistic about the kind of terrorist threat that we face currently Prof Paul Wilkinson, Terrorism expert

Ricin was not the obvious weapon to cause mass casualties, he said, but added: "It is very worrying that apparently a large quantity of this very dangerous poison was being held in such close proximity to a variety of different targets in London.

"I'm sure the police investigation will be carried forward with great energy because it is important we find out what these people were planning and who was behind it.

"It is correct to be realistic about the kind of terrorist threat that we face currently, and I think that the government has already indicated that they regard the al-Qaeda threat and the threat from its affiliates from around the world as extremely dangerous.

"I think those who follow what the terrorists have been doing in different parts of the world will see why that assessment has been made."

He concluded: "It is very important that people remain extremely vigilant and they co-operate to the maximum with the police if they have any information which could be useful in the investigation."

European cities threat

Sir Timothy Garden, former assistant chief of defence staff said it was no surprise the discovery had been made.

Speaking on BBC News 24, he said: I think we've been having plenty of signs from Special Branch in London that we are under threat.

"It's difficult to know where the next terrorist threat will happen and what sort of event it will be, they can choose from anywhere in the world as we saw in Bali, or before that in 11 September.

I think Tony Blair is absolutely right to warn the public to be on their guard, but not to panic and not to feel overly threatened Prof Tony Mason, International security expert

"Everybody knows that Western European cities are at threat.

"We should be to an extent reassured that a lot of work is going on by the intelligence services, of which we will only see the tips of the iceberg.

"This looks as though it's going to be one of those tips of the iceberg.

"People mustn't be worried about appearing foolish - if you think there is something of concern, some different activities, some suspicious things, then report it and let it be investigated.

"This is a serious problem throughout the world, and the more information that we have the more the intelligence services can protect us."

Assassination weapon

Richard Sullivan, a bio-terrorism expert, said ricin was currently classed as a Schedule B toxin by the Center for Disease Control in Atlanta, US.

He told Sky News: "It's mainly been used in the past as a biological weapon for assassination purposes. It hasn't been deployed as a weapon for mass destruction before.

"It's an extremely powerful toxin."

Professor of international security at Birmingham University, Tony Mason, said Osama Bin Laden may have been behind any potential terrorist threat arising from this incident.

Speaking on News 24, he said: "Bin Laden quite recently threatened the populations of Britain and Australia and France and one or two other countries, in addition to the US.

"And we also have to remember that before any attack was made on the training camps in Afghanistan last year, up to 25,000 people had already been trained and scattered worldwide.

"So there is a threat and I personally believe this is not a government inspired incident.

"I think Tony Blair is absolutely right to warn the public to be on their guard, but not to panic and not to feel overly threatened."