The UK must enforce "equality, participation and interaction" to avoid US-style segregation, the head of the Commission for Racial Equality says.
The young are 'more exclusive' than older people, Mr Phillips said
Failing to do so could lead to people living in a New Orleans-style Britain of passively co-existing communities, Trevor Phillips warned.
It was a worrying fact that "younger Britons appear to be integrating less well than their parents", he said.
The comments were "very pessimistic" said his predecessor, Lord Ouseley.
Mr Phillips was speaking at a Manchester Council for Community Relations lecture.
He cited economic and racial divisions in the US which, he said, were highlighted by Hurricane Katrina.
"Residentially, some [UK] districts are on their way to becoming fully fledged ghettos - black holes into which no-one goes without fear and trepidation, and from which no-one ever escapes undamaged," he said.
"If we allow this to continue, we could end up in 2048, a hundred years on from the Windrush, living in a New Orleans-style Britain of passively co-existing ethnic and religious communities, eyeing each other uneasily over the fences of our differences."
Mr Phillips said America's "segregated society" had been caused by a "failure to act until they were in too deep to get out of the state they are now in.
"That is why, for all of us who care about racial equality and integration, America is not our dream but out nightmare."
Mr Phillips also warned that communities were being left "marooned outside the mainstream".
These would "steadily drift away from the rest of us evolving their own lifestyles, playing by their own rules and increasingly regarding the codes of behaviour, loyalty and respect that the rest of us take for granted as outdated behaviour that no longer applies to them".
And he warned that levels of racial segregation in Britain could create a "fertile breeding ground for extremists".
"It also remains true that younger Britons are more exclusive than older Britons," he said.
Lord Herman Ouseley, a previous head of the CRE, said: "I think he's right not to warn us to be complacent because there are difficulties and problems in some parts of the country.
"But to be so sweeping to suggest that in Britain we're not talking to each other, we're not aware of differences, we're not getting on in many parts of the country, I think is totally wrong."
Home Office Minister Paul Goggins welcomed the issue of integration being raised.
"We have very good community relations in this country and its something to be proud of and to build on," he told BBC News.
"But he's right to say we need to do yet more to make sure our communities are fully integrated."
Earlier on Thursday, Mr Phillips told BBC Radio 4's Today programme that schools should be leading the way in terms of integration but research had shown they were in fact slightly more segregated than their wider neighbourhoods.