The foiling of an alleged chemical bomb plot in Britain vindicates the government's continued warnings about the terror threat, David Blunkett says.
Osmium tetroxide is not known as a chemical weapon agent
Al-Qaeda sympathisers were believed to be targeting civilians in London.
The plot was thought to involve detonating a combination of explosives and a chemical called osmium tetroxide.
Britain's top policeman, Metropolitan Police Commissioner Sir John Stevens, has reiterated his belief that the UK was "in a state of real danger".
Underlining his concerns about the terrorist threat, he said: "There is the chance that someone will slip through - it's my job to ensure they don't
Sir John Stevens told The Spectator magazine it would be a dereliction in his duty if he "didn't engage the main means of combating terrorism ... the population".
'Our only protection'
Mr Blunkett told BBC Radio 4's Today programme that recent events should counter claims in people's minds that "we exaggerated" the threat.
"All of us, for two-and-a-half years have been indicating that that is precisely what the network called al-Qaeda, in its loose form, are actually about," he said.
The public should be "praising and being very grateful that we have the security and counter-terrorism services we do because they are doing a first-class job", he said.
"They have got my whole-hearted backing because this is the only protection we really have."
Meanwhile, the home secretary has made some changes to the way his department deals with counter-terrorism.
Hazel Blears, who is minister for policing, crime reduction and community safety, will now be responsible for counter-terrorism which was previously the concern of Beverley Hughes before she resigned as immigration minister last week.
Ms Hughes' replacement, Des Browne, will now concentrate more on asylum and immigration following a series of controversial scandals for the government in this area.
The target of the alleged chemical and explosives plot was thought to be areas in which there would be concentrations of people, possibly within a confined space.
Alastair Hay, Professor of environmental toxicology at Leeds University, said osmium tetroxide was a rare catalyst and could potentially make an explosion occur more rapidly.
Laboratory chemical which requires precautions when handled
Used in scientific experiments, not known as a chemical weapon agent
Toxic and irritant, even in small amounts
Direct contact causes skin and eye damage
But he told the BBC it would have to be obtained from a specialist chemical supplier and it did not fit the profile of a typical chemical warfare or dirty bomb agent.
"It would not be in the same category as some radioactive substance which would continue to emit radiation and cause a problem in terms of clean up," he said.
Security expert Dr Sally Lievesly said terrorists were well aware of the psychological impact a chemical bomb would create.
"The emergency services would be faced with a terrible scene. They would have to kit out, they'd be delayed and the injuries they'd be seeing would be very bad, so their job is a very difficult one," she told BBC One's Ten O'Clock News.
"With these types of attack, if the public are not prepared, this then becomes a weapon of psychological terror."
The UK has been on a high state of alert since the bombings in Madrid on 11 March.
Sir John has in the past said a terror attack on London was inevitable, but Mr Blunkett has tried to play this down.
Tory leader Michael Howard has written to Tony Blair asking him to clarify his position on the need for a dedicated minister for homeland security.
In his letter, Mr Howard says it is not clear from recent comments who is responsible for combating terrorism.
"It is unacceptable in the current climate that there is confusion over who is in charge of counter terrorism, and I ask you to make clear your position on this vital issue as a matter of urgency," he said.
Earlier, a Whitehall official told the BBC that, had this plot succeeded, it would have been the most serious attack on Britain in many years.