Almost a week later, and after a downgrading of the terrorist threat, what do ordinary voters now think of those excitable early briefings by John Reid, the home secretary, and Scotland Yard's dramatically voiced belief that it had foiled "mass murder on an unimaginable scale"? Are people still sceptical?

As it turns out, the prime minister would be heartened by the views of the individuals on his doorstep. Unfortunately, the largely sympathetic crowds outside Downing Street are overwhelmingly made up of non-voter tourists. A Brazilian who has lived in London for five years mentions his innocent countryman, Jean Charles de Menezes, shot dead after botched intelligence following 7/7 last year, but says that, relatively speaking, he trusts the authorities more here than in his home city of Sao Paulo. Similarly, American visitors to the capital say they are impressed with the "very British, very discreet" security and the post-alert calm of the population. But they are bewildered by the sceptical reaction of some Britons to what they see as a war.

"We've been looking at your museums," says John LeClaire, from Boston, holidaying with family and friends. "In the first world war there's this blindly patriotic joining of this war that is in a sense pointless. In the second world war there is uniform support for the government once Chamberlain is got rid of. Now you have, what, about 20% of the people who think terror plots are a conspiracy? That's an extraordinary evolution."

"It didn't cross my mind that this was a conspiracy," says Dogan Arthur, also from Boston. "It would show that terrorism is working if people think it's a conspiracy." He adds: "It's remarkable how international a city London is. It's interesting what a presence the Middle East has in London. I don't remember that 10 years ago. There's nowhere in the US where you would get that sense of being in a sea of Muslims."

Thirty five miles north of Downing Street and the average American tourist really would be startled by Bury Park in Luton, a quintessential 1920s English suburb now predominantly populated by British Muslim families. Halal butchers, grocers selling piles of fresh watermelons and fashion stores offering "wedding sarees, langhas and fabrics" line the main street.

"In my opinion it is a cover-up because of what's going on in Lebanon," says Munir Khan. "When you turn on the TV you see innocent people getting killed. This [plot] distracts from that."

A moderate Muslim who has been a member of the Labour party for nearly 20 years, Khan quit about eight months ago to join the Lib Dems. He does not trust the evidence coming out of Pakistan in relation to this latest alleged terror plot. "The Pakistan government will say anything for money," he says. "If the UK government gives them money to say something, they will say it."

Suspicion and (often internet-fuelled) talk of conspiracies is no longer the prerogative of the young, it seems. According to Khan, it has been noticeable that older Muslims, traditionally engaged in mainstream politics in a way that their children are not, have talked openly of their anger and distrust of the government in recent months.

Scepticism about the plot is shared by many in the area and not just by Muslims, says Qurban Hussain, a local resident and the deputy leader of Luton borough council. "People are definitely sceptical. They are not sure whether these claims are just to clamp down on British Muslims. Is it scaremongering tactics by the government or another reason to harass more innocent people?

"It's a perception held by a lot of my constituents of all backgrounds. When you look back on the WMD, the information was wrong. Then we have the case of Jean Charles de Menezes. We picked up the wrong person altogether. Then the raid in Forest Gate in which a man was shot. There are so many cases people can refer to. It makes them feel they cannot trust the government."

Reassuringly for Labour, the overwhelming majority of people of all faiths I speak to in London, Luton and further up the M1, give a pragmatic, sensible-sounding "wait and see" response to questions about whether they believe that this alleged terror plot is a genuine threat.

"It is in the government's best interests to look after the people of this country. If they hear of a threat, they have every right to close things down," says Usman Hussain, 21, inside a barber's in Bury Park. On the window is a poster advertising a talk on the "crisis in Lebanon". The star guest speaker is billed as a member of Hizb ut-Tahrir, the radical Islamist group. Does Usman believe there was a terrorist plot to blow up planes? "I don't really know. We've been given no evidence yet. It could go either way."

But this general pragmatism is heavily steeped in scepticism everywhere I go, and not just among British Muslims. Jerry Thornton, from Wiltshire, is with the tourists outside Downing Street. "There is so much we don't know. It [the government] is such a secretive organisation. They are all colluding together. Some of it's for our own protection, but I believe a lot of it is spin. I accept during the investigation they can't tell the whole truth but we'd like to know exactly what happened and how it was foiled."

Stopping at the motorway services just north of Luton, John Jeffreys is unsure whether he trusts the government's line. "It's difficult to know. A lot of these terror alerts seem to coincide with an announcement about ID cards for instance. This time there obviously was some sort of plot but we don't know how significant it was. I don't trust the government at all. There's no doubt that Blair lied about the weapons of mass destruction before Iraq."

"It's propaganda, isn't it?" chips in his mate, Mick Perrone, 31. "It gets the whole nation on alert."

What can the government do to restore trust? In a time of endlessly slippery conspiracy theories, how can it show that the terror plot was genuine? "They must come up with the proof," says Khan. Other Muslim voters argue that the first step the government must take to restore levels of trust is to reform its foreign policy. Again and again, Muslim voters point to what they see as the government's "double standards" in dealing with other Islamic countries and disputes in Iraq, Afghanistan, Palestine, Kashmir and now Lebanon.

Muslim voters say they are also angered by the government's - and George Bush's - use of the term "Islamic terrorism". "Why Islamic? Look at Northern Ireland. Who was saying 'Christians' there?" says Khan.

By chance an Irish family on holiday from Belfast pull into the services on the M1 while I'm there. "This [plot] could be make-believe, so the government can say, 'Look what we're doing to fight the terrorists,'" says Joanne Burrows. "There must have been something to arrest 23 people, but plenty of people have done time in Northern Ireland for doing nothing".