The prime minister made clear he was moving towards committing Britain to the most daunting military campaign since he came to power.

"You can be in no doubt of our determination to act to make sure those responsible are brought to account," he said at a joint press conference in Washington last night with President George W. Bush.

He said Britain would stand "side by side" with the American people as they had done for the UK during the second world war.

"We give you our solidarity, our sympathy, our support."

Mr Bush, who was preparing to tell US forces to "be ready" in a speech to Congress, said Mr Blair had been a "true friend".

Earlier, Mr Blair visited New York to attend a service commemorating the 200-300 British citizens missing in the ruins of the World Trade Center.

In his address to the inter-faith service attended by 1,500 at Manhattan's St Thomas's church, he said: "After the terrible events of last week there is shock and disbelief; but throughout the world, there is a profound sense of solidarity; there is a surging of the human spirit. The bonds between our countries, for so long so strong, are even stronger now."

The Queen, in a message read at the service by Sir Christopher Meyer, British ambassador, said: "These are dark and harrowing times for families and friends of those who are missing or who suffered in the attack. Grief is the price we pay for love."

The service, half a mile from the site of the attack, was attended by former president Bill Clinton and Kofi Annan, UN secretary-general. Mr Blair chose a passage from The Bridge of San Luis Rey, written by Thornton Wilder in 1927 about a bridge collapse in Peru in which five died. "There is a land of the living and a land of the dead, and the bridge is love," he read.

En route to the US, Mr Blair spoke by telephone from his Boeing 777 with President Mohammad Khatami of Iran.

"That in itself was a remarkable conversation in the sense that not merely did he give his full solidarity in terms of his outrage at what had happened in the US and his strong condemnation of terrorism, but also how important it was that out of this we build a new relationship between our two countries," Mr Blair said.

Many Iranians had been murdered by Afghan-based terrorists, and there was a "strong sense in which the world of Islam is wanting to join with us in common cause against terrorism". He said: "This was a conversation I would not have imagined having some weeks ago."

After breakfast in Paris with President Jacques Chirac, at which both pledged "appropriate" military action, Mr Blair said he hoped "in the next few days we demonstrate as a world our complete solidarity in this fight".

He later urged people not to jump to conclusions about timing. The US was struggling to draw up plans that will carry broad-based support and a fair chance of success. Mr Blair said: "We have no option but to act, but of course this is a huge and heavy responsibility, that is why we have to deliberate carefully before taking it. It is important to make sure that the action we take is effective."

It was "even more difficult for those who have to carry out the action on the ground. We are extremely lucky in Britain to have some of the finest armed forces anywhere in the world."