Home Secretary David Blunkett says a warning on terrorism was toned down in order to avoid "creating unjustified panic and disruption".

The draft statement was released in error on Thursday, but withdrawn minutes later and replaced with a revised text.

The original release warned of a possible chemical or nuclear terrorist attack on the UK using a "dirty bomb" or poison gas.

The later statement contained a more general warning of "ever more dramatic and devastating" terror attacks.

Mr Blunkett said the initial statement was released as a result of "a simple clerical error".

Maybe they will try to develop a so-called dirty bomb, or some kind of poison gas; maybe they will try to use boats or trains, rather than planes

Original warning

But he said the underlying message - the need for vigilance in the face of the threat from al-Qaeda - was more important than the mistake.

He said he took responsibility for errors in his department.

He said: "I want to use language that people can understand but which is also an accurate reflection of the information presented to me at any time.

Duty

"That and that alone is the explanation for the difference between the two drafts."

Mr Blunkett said his duty was to give an assessment of any threats faced while also balancing that "against the risk of creating unjustified panic and disruption which would itself give the terrorists the victory they crave".

He said: "No one should be in any doubt that the protection of the public is my highest priority as Home Secretary.

"Where tough measures are needed, I will not flinch from them. But I will balance those measures by safeguarding the very liberties that they are designed to protect."

Downing Street said the administrative slip-up should not divert attention from "a serious and ongoing threat from al-Qaeda".

Poison gas

The draft statement warned that al-Qaeda could strike with traditional terror tactics or new, "surprising" methods.

"Maybe they will try to develop a so-called dirty bomb, or some kind of poison gas; maybe they will try to use boats or trains rather than planes," it said.

The second statement said: "If al-Qaeda could mount an attack upon key economic targets, or upon our transport infrastructure, they would."

The warnings came in the foreword to a summary of anti-terrorist measures taken by Britain in recent months.

Both statements urged people to remain vigilant to the continuing threat of Irish and international terrorism.

Wider threat

And Prime Minister Tony Blair's spokesman said that was the key message which should not be overshadowed by the administrative mistake.

The two statements released by the Home Office

He said: "What it says is that there is a serious and ongoing threat from al-Qaeda, that that is confirmed by intelligence reports from various parts of the world, that that threat applies just as much to this country as to others.

"And as we learned on September 11, this is a qualitatively different kind of threat from previous terrorist threats."

BBC correspondent Frank Gardner said the truth was that Britain's security services faced the dilemma of believing the threat of an attack was high, but not knowing where it might come from.

"They want to warn the public to be vigilant but at the same time not panic them," he said.

'Not inaccurate'

Professor Paul Wilkinson of the Centre for Terrorism Studies at St Andrews University said it was a "possibility" that there was a threat from a "dirty bomb" attack in the UK.

So-called "dirty bombs" scatter deadly radioactive material using conventional explosive devices.

While not as immediately destructive as traditional explosives, they could ultimately prove far more devastating in terms of casualties, as they have the potential to spread radioactive material over a wide area, possibly leading to cancer and radiation poisoning.

Professor Wilkinson told BBC Radio 4's Today programme that the original home office warning was "not inaccurate".

"I suspect that really the reason for its withdrawal was that they did not want to highlight specific threats.

"They wanted to point out that the threat was from a whole variety of tactics," he added.