After decades of campaigns by Irish Republicans and, most recently, Islamist militants, Britons have become used to the daily threat of terrorism.
But in a warning that the stakes have been raised – and just days before world leaders gather here for the Group of 20 meeting – a warning was given this week that a so-called dirty bomb on a British city is more likely than ever.
The government alert accompanied the launch of a major new antiterrorist strategy that encourages ordinary citizens to offer Britain an additional layer of security.
The new approach aims to train some 60,000 retail, hotel, and service industry staff to recognize terrorist threats. In addition, more resources will go into blocking access to information posted online on how to stage terror attacks.
Most significant, as part of a broader ideological offensive against terrorism and amid growing concern that alienated Muslim youths are being recruited by terror groups, the government will allocate funds for influential groups and individuals in Britain's Muslim community who speak out against extremism.
The 167-page document, regarded as the frankest assessment yet of the threat facing Britain, asserts that there is a need to "challenge those who reject the rights to which we are committed, scorn the institutions and values of our parliamentary democracy, dismiss the rule of law, and promote intolerance."
The document made headlines with a stark warning that changing technology and increased illegal transportation of chemical, radiological, and biological materials make the prospect of terrorists assembling and exploding a dirty bomb more realistic.
Currently, the risk of a terrorist attack taking place in Britain is said by the authorities to be "severe" – meaning that an attack is highly likely.
The threat of home-grown militants importing technology to construct improvised explosive devices of the type used against British and US forces in Iraq and Afghanistan was also highlighted in the report, called "Contest Two," which updates the previous "Contest" strategy developed in 2003.
The strategy's launch, however, was overshadowed by a continuing rift between the British government and Britain's largest Muslim group, which accused ministers Thursday of wanting to "undermine its independence" by demanding one of its leaders be removed from office.
The government, which has previously funded the Muslim Council of Britain, broke off relations earlier this month after it called for the resignation of a senior council official after he allegedly called for violence against Israel.
The concerns outlined in the document corresponded with a separate report this week by the British broadcaster, Sky News, which cited Pakistani intelligence sources saying that more than 20 young Britons had returned home after being trained by groups linked to Al Qaeda and the Taliban.
British Home Secretary Jacqui Smith named next week's G-20 meeting as a prime target for terrorists and told the BBC that Britain could no longer just rely on intelligence services and the police to stop the threat.
In remarks seen as directed at Britain's Muslim population, Ms. Smith said that it was up to people in all communities to challenge those who espouse radical beliefs and reject aspects of British life such as a parliamentary democracy.
"We should all stand up for our shared values and not concede the floor to those who dismiss them," she told the BBC.
Her remarks, as well as the general tone of the strategy, mark a shift in Britain's approach to tackling terrorism. Much of the effort has gone into disrupting terrorist operations. But now, officials will focus more on challenging those who attempt to undermine British society's established values.
In what is regarded as reflecting more nuanced thinking by Britain's intelligence services, the report acknowledges the role played by perceptions of British foreign policy in the Islamic world, and how that anger has been fueled by events in Iraq and Afghanistan, as well as perceived inaction over the plight of Palestinians.
However, the paper argues that such grievances do not always lead to radicalization, noting that social and psychological factors, as well as individual conflicts of identity, can be important. Playing down the role of radical preachers, who have in the past been blamed for radicalizing young British Muslims, it highlights instead the role of charismatic individuals and peer groups.
The strategy does offer some cause for optimism, suggesting that the "severe" threat level could be downgraded over the coming year.
Al Qaeda was also said to be short of money. The group is also likely to fragment, according to the assessment, and Britain and other nations might be targeted by new "self-starting" terrorist groups or individuals.
Brooke Rogers, a lecturer in risk and terror at King's College London, described the British government's warning of an increased risk of a dirty bomb attack as "brave."
Dr. Rogers added, "It needed to be said, although they had to be careful how they said it, but it also needs to be accompanied with a more joined-up program so that people can actively take steps on how they can reduce the risk as much in their own lives."
She welcomed other aspects of the strategy, particularly its emphasis on more actively challenging those who express views that are contrary to British values.
"A big question in recent years has been whether or not the British system of celebrating multiculturalism has actually worked," Rogers says.
"To some extent, the British identity has been eroded, and what I think the government is now trying to do is draw a firm line around what it is to be British in terms of believing in democracy and freedoms."