Mr Grieve said Britain had failed in creating a cultural 'melting pot'
British multiculturalism has left a "terrible" legacy which has allowed extremists to flourish, shadow home secretary Dominic Grieve has warned.
A type of "cultural despair" has led "long-term inhabitants" and newer arrivals to feel alienated and unsure of UK values, he told the Guardian.
Mr Grieve, speaking on the eve of the Conservative Party conference, argued this had led to support for extremism.
He also warned against downplaying Britain's Christian heritage.
Mr Grieve told the Guardian: "We've actually done something terrible to ourselves in Britain.
"In the name of trying to prepare people for some new multicultural society we've encouraged people, particularly the sort of long-term inhabitants, to say 'well your cultural background isn't really very important'."
The vacuum created by multiculturalism has encouraged support for extremists on both sides, he argues.
The shadow home secretary went on to say multiculturalism was inspired by the "understandable" desire to make people feel comfortable.
But he added: "The idea behind it was [to] create the melting pot. But the melting pot needs the ingredients of people's confidence in themselves as they come together. And if it isn't there I think we've done ourselves huge damage."
Mr Grieve also said the part played by Christianity in Britain should not be ignored.
"The role of Christianity is really rather important. It can't just be magicked out of the script. It colours many of the fundamental viewpoints of British people, including many who've never been in a church."
It is not the first time a Tory home secretary has spoken out against multiculturalism.
In 2005, the then shadow home secretary David Davis called on the government to scrap the "outdated" policy, saying that allowing people of different cultures to settle without integrating let the "perverted values of suicide bombers" take root.
Mr Davis said he agreed with Trevor Phillips, the chairman of the then Commission for Racial Equality, who a year earlier argued multiculturalism belonged to a different era.
Mr Phillips said all citizens should "assert a core of Britishness".
Asked about Mr Grieve's comments on BBC One's Andrew Marr show, Conservative leader David Cameron said he agreed that "state multiculturalism" had been the wrong approach.
Mr Cameron said: "What he said was that state multiculturalism - the idea that as welcoming people into our country and keep them all in silos, and treat British Muslims as Muslims, rather than as British citizens, treat British Jews as Jews rather than as British citizens - that is wrong.
"I think trying to integrate more, trying to bring people together more, trying to build a strong British identity for the future, I think that's absolutely right."
Mr Grieve's comments come as an ICM poll for the Guardian suggested Labour had narrowed the gap on the Tories.
The poll put the Conservatives on 41%, Labour on 32% and the Lib Dems on 18%.
A ComRes survey for the Independent on Sunday last weekend put the Tories on 39%, Labour on 27% and the Lib Dems on 21%.
The Tory conference will open on Sunday in Birmingham, with a discussion on the economy involving shadow chancellor George Osborne and shadow foreign secretary William Hague.
Leader David Cameron's speech closes the conference on Wednesday.