Tony Blair has defended President Bush's controversial visit to the UK.

The prime minister said Britain had a duty to stand side by side with the American president.

"This is absolutely the right time for him to come," Number 10 said on the eve of the Bush visit.

"The visit will demonstrate the depth and the breadth of the relationship between the two countries."

Number 10 highlighted the range of inter-dependence: a million people are employed in the UK by US firms, 38,000 US students are currently studying here and more US citizens live in the UK than in any other country outside the US.

The president and his wife will arrive in London tomorrow for the first state visit of a US president for over 80 years.

Around 14,000 police and 700 US special agents will be on duty for what appears set to be one of the most high security events ever to be seen on the streets of the Capital.

Number 10 welcomed the chance for a "vigorous debate" with protestors and offered a novel take on the issue of civil rights.

"There's a balance to be struck in a democracy between people's right to protest and people's right to go about their business - including visiting dignitaries," said the official spokesman.

Security in London was stepped up over the weekend, with armed police patrolling the area around Whitehall and the Mall.

Tourists wishing to walk down Whitehall were being screened and frisked ahead of the president's trip.

Bush, who will be staying at Buckingham Palace, will visit the Banqueting House in Whitehall and Westminster Abbey.

He will also attend a Downing Street lunch cooked by celebrity chef Nigella Lawson and is due to visit the prime minister's Sedgefield constituency on Friday.

The total cost of the visit is likely to exceed £19 million - with the British taxpayer funding £4 million in policing costs.

Fears over an al Qaeda attack during the visit have led to an unprecedented security operation in London.

Intelligence suggests that the terror group is "on the move" and may attempt a "spectacular" attack on the UK mainland during the president's visit.

The Home Office confirmed yesterday that Britain is now on a heightened state of security alert.

The president is also set to face a series "Stop Bush" protests during his three-day visit to London.

Bush has welcomed the demonstrations despite the fact that they appear set to overshadow his visit.

Speaking to Breakfast with Frost on Sunday, the president said he was unfazed about the prospect of demonstrations.

"Freedom is a beautiful thing. I value going to a country where people are free to say anything they want to say," he said.

"Secondly, I would say that I understand you don't like war, and neither do I.

"War is my last choice, not my first choice, but I have [an] obligation as the president to keep our country secure."

More than half of all British voters back the planned demonstrations against President Bush, a poll published over the weekend revealed.

The You Gov poll for the Sunday Times also found that 60 per cent of Britons believe the president is a threat to world peace.

Thirty seven per cent of those polled believe he "is stupid" and 33 per cent class him as "incoherent".

Anti-war MPs spent the weekend stoking up the pressure on the prime minister.

Robin Cook warned ministers that the British government should not allow itself to be "used" in the looming presidential election.

Whilst he accepted that the White House had not foreseen the scale of the protests, Cook said Bush was set for "the mother of all photo opportunities" at Buckingham Palace.

He said the president's campaign managers would seize on the "pomp and circumstance of an official state visit".

"When you think back to how closely we worked with President Clinton and how helpful President Clinton was to British interests, particularly on Northern Ireland, I do find it mystifying that the same Royal visits committee that decided President Clinton doesn't merit the honour of an official state visit should now decide that President Bush does," he added.