A leading defence think-tank has said that multicultural Britain is an easy target for attacks by militant Islamists because its aims, values and political identity are divided.
In a report strongly rebutted by the government, the Royal United Services Institute (RUSI) said: "We look like a soft touch. We are indeed a soft touch, from within and without."
How much to integrate Britain's ethnic communities, particularly its 1.8 million Muslims, has been a hot political issue since four British Islamist suicide bombers killed 52 people on London's transport system in July 2005.
The attacks sparked a debate on whether Britain's policy of avoiding imposing a single British identity and instead promoting a multicultural society had led to segregation of ethnic minorities.
The report, based on the findings of former military chiefs, diplomats and analysts, concluded: "The country's lack of self-confidence is in stark contrast to the implacability of its Islamist terrorist enemy."
"The security of the United Kingdom is at risk and under threat," it said.
The report called for the creation of a new cabinet committee to oversee security policy. It said another parliamentary committee should seek to build consensus and identify security weaknesses.
"Islamist terrorism is where people tend to begin. The United Kingdom presents itself as a target, as a fragmenting, post-Christian society, increasingly divided about interpretations of its history, about its national aims, its values and in its political identity," the RUSI report said.
"That fragmentation is worsened by the firm self-image of those elements within it who refuse to integrate."
The report said "lack of leadership from the majority, which in misplaced deference to 'multiculturalism' failed to lay down the line to immigrant communities" had undercut those within them trying to fight extremism.
That provoked a robust response from the government, with a cabinet office spokesman saying: "The government rejects any suggestion that Britain is a soft touch for terrorists".
"We have a detailed and robust strategy for countering international terrorism," he added.
Labour parliamentarian Keith Vaz said the report was wrong to blame multiculturalism for fostering terrorism.
"I think one of the problems with this report is that they've not actually looked at our multicultural society to see what benefits it has given the country. It hasn't been a soft touch for terrorism," he told BBC Radio.