The tragedy that followed the Moscow theatre siege and other recent outrages reconfirms the need for terrorism to be defeated, Prime Minister Tony Blair has told MPs.

Mr Blair said the hostage drama and terrorist attacks like the bomb blast in Bali were "brutal and horrifying reminders of this new form of terrorist extremism".

The war on terror must continue

Iain Duncan Smith

"A deadly mixture of religious and political fanaticism is being pursued by those who have no compunction about taking human lives, no matter how innocent and little about losing their own.

"The only answer is both to defeat them by security, intelligence and policing but also to take head on, especially within the Muslim world, their perversion of Islam in the cause of extremism."

He was making a statement to the House of Commons in the wake of the siege which ended with a heavy loss of life.

British hostages

Describing the deaths of the theatre hostages as a tragedy, Mr Blair stressed that there were "no easy, risk-free, safe solutions".

He argued the Russian authorities had needed to act when terrorists started to kill the hostages.

All three British hostages held in the theatre escaped unharmed.

Peter Low, from north London, released early because of health problems, and wife Sidica and son Richard surviving the storming of the building.

The reunited family arrived back in the UK on Monday evening and said they were "extremely happy" to be back safe and well.

Peter Low said: "We are immensely grateful for the efforts of everyone to secure our release and on this day of national mourning in Russia we offer our sincerest condolences to the families who have lost loved ones.

"Our special thanks to the British ambassador and all his staff at the British embassy in Moscow for their kind hospitality and thoughtfulness during the past few days."

UN resolutions

Earlier, Mr Blair was reporting back to MPs on last week's European summit, where EU leaders were "all agreed" on the need to ensure Iraqi dictator Saddam Hussein had no weapons of mass destruction.

Mr Blair said he had "no doubt" action must follow any future breaches of UN resolutions by Iraq.

There are still worries about many hostages

His words prompted Conservative leader Iain Duncan Smith to argue that the Moscow events proved that "the war on terror must continue and those who say we can take a break from it are fundamentally wrong".

The prime minister, Mr Duncan Smith and Liberal Democrat leader Charles Kennedy all expressed their condolences to the Russian people.

The Tory leader sympathised with the "huge and terrible dilemma" facing Russian President Vladimir Putin.

'Known links with al-Qaeda'

Later Foreign Secretary Jack Straw told an influential cross-party group of MPs that the threat of international terrorism represented the "greatest strategic challenge to the civilised world".

He also stressed that "there might be" a legally valid basis for the commencement of military action against Iraq without a new UN resolution.

Giving evidence to the Commons foreign affairs select committee, Mr Straw said both the Chechen rebels responsible for the theatre attack and the Indonesians, believed to be to blame for the Bali bomb blast, were "known to have links with al-Qaeda" although the precise nature of the links was not certain.

"But the fact that well over 300 people have been killed and many more injured in terrorist outrages in the space of two weeks should alert us to the continuing threat that we all face from this type of terrorism."

Mr Straw was again forced to go on the defensive about how much he knew about imminent threats against Bali, stressing he had not received intelligence "sufficient to justify the use of the word warning".

"It's immensely difficult," he said. If we were to react to every piece of intelligence, the world would seize up - we'd have done the terrorists' job for them."

Sleeping gas

He was speaking as Mr Putin said he would be giving the military new powers to fight terrorism, two days after special forces ended the Moscow theatre siege.

Anger is growing over the tactics used to overpower Chechen rebels who took over the theatre last Wednesday, particularly the use of an unidentified sleep-inducing gas which ended up killing indiscriminately.

About 118 hostages are now known to have died - all but one from gas poisoning. Doctors are warning the death toll could rise further, with hundreds more still in hospital and some critically ill.

New moves have been taken after the Bali bomb

Mr Blair's warning about the continued threat of terrorists came as Home Secretary David Blunkett moved to outlaw the radical group suspected of being involved in the Bali bombing.

Four groups believed to be linked to al-Qaeda, including the militant Indonesian group Jemaah Islamiyah, would be banned under orders put before Parliament on Monday.

There area already 21 international organisations banned and under the Terrorism Act 2000 and Mr Blunkett wants to add another four to the list.

Mr Blunkett said: "Proscription is not a power to be used lightly".

Outlawing the four groups would send a "clear message" that the UK was committed to playing a leading role in the international campaign against terrorists, said Mr Blunkett.

The move is due to be debated by MPs and peers later this week, who will both need to approve the plans and would ban membership, funding or support of the groups a criminal offence.

UK banks have already been told to freeze Jemaah Islamiyah's assets.