Brown's message to terrorists: 'We will not be intimidated. Terror will not undermine our way of life'

Gordon Brown delivered an uncompromising message to Islamic extremists behind the car bomb plots in London and Glasgow yesterday that Britain "will not yield" to terrorism, but warned of stringent security measures and continued disruption across the country.

There was speculation the bombers may have been trying to put pressure on the new Prime Minister after the departure of Tony Blair to speed the withdrawal of British troops from Iraq. But Mr Brown said he would not change course.

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"We will not yield, we will not be intimidated," the Prime Minister said. "It's very important that people carry on living their lives as normal. It's very important that we, the British people, send a message to terrorists that they will not be allowed to undermine our British way of life."

The alert state was raised to "critical", signalling the threat of further terrorist attacks was imminent, after Mr Brown chaired a meeting of Cobra, the emergency planning committee.

Mr Brown said tighter security across Britain could cause disruption with longer queues at airports, and police checks in "crowded places" such as city centres and rail stations. Drivers may also be delayed for roadside checks for explosive devices.

MPs' staff and government officials returning to the Palace of Westminster and Whitehall for work today face lengthy queues, as bomb checks are imposed on workers in spite of their security passes.

Mr Brown said: "I've got to appeal to members of the public to understand that when we've had an attempted attack an airport, the security measures have to be increased. Whether it's checks as people go into the airports or whether its also more police patrols, or whether it may be barriers people have to come through, these will be on the basis of an assessment made on the ground for particular airports.

"We have got to take measures in crowded places and you will see a greater police presence and you will see in some cases further measures to enhance security there. And people may expect checks of cars."

The terrorist attacks forced Mr Brown to postpone a statement scheduled for today on sweeping constitutional changes, including giving Parliament a right to a vote on Britain going to war. He is planning controversial measures in an anti-terrorism Bill inherited from Mr Blair. The Bill will reopen the row over 90-day detention of suspects without charge. But senior officials said Mr Brown wants to avoid it being seen as a "knee-jerk" reaction to the weekend attacks.

Mr Brown has already had talks with David Cameron, the Tory leader, on anti-terrorism legislation to try to seek a consensus. William Hague, the shadow foreign secretary, hinted the Tories may be prepared to move on their objection to an extension of 28-day detention.

The Tories suggested privy councillors should investigate using phone tap evidence in terrorist cases, which has been resisted by the intelligence services, and Mr Brown has ordered the review There is also cross-party support for suspects to be questioned after they are charged.

Mr Brown rejected claims that the war in Iraq had contributed to the terrorist attacks in Britain. "The terrorist threat is long term and it's sustained. It is about those people who are essentially violent extremists who have a grievance against society, particularly against the values that we represent and the values that decent people in all religions represent. It is their intention to inflict the maximum of damage in any part of the world to make their propaganda point. Of course we want greater peace and security in Iraq and Afghan-istan. Making progress in the Middle East with Palestine and Israel will make a difference."

He added: "Anybody I talk to, a leader in any part of the world, knows we're dealing with a long-term threat unrelated in detail to one specific point of conflict in the world, but is a general view that is held by a group of terrorist extremists about the shape of the world they want to create."

Mr Brown compared the battle against Islamic extremism to the ideological battle by the West against Communism. "We've got to separate those great moderate members of our community from a few extremists who wish to practise violence and inflict maximum loss of life in the interest of the perversion of their religion." It would require action to win "hearts and minds" as well as security and military measures, he said. "That's why the cultural effort, similar to what happened during the Cold War in the 1940s, 1950s and 1960s, when we had to mount a propaganda effort to explain to people that our values represented the best of commitments to individual dignity to liberty and to human life being taken seriously. And that's what we are going to have to talk about in the next few years."

The celebrity concert in memory of Diana, the Princess of Wales, went ahead at Wembley with the Princes William and Harry present.