Immigrants coming to the UK today are more likely to be young, single and competing for better jobs, the head of the C ommission for Racial Equality said.
Trevor Phillips said the most recent influx of migrants, including those from EU Accession states such as Poland, posed "some major new issues".
He said they brought with them their own institutions and were settling in greater numbers in smaller areas.
Speaking at the Royal Geographical Society, he said: "There are some features of this migration which make it - from the point of view of the average citizen - very different to the post-Empire wave.
"The first is pretty obvious if you go into any Starbucks, or walk past a building site. These people come to work, and to earn.
"That doesn't mean that settled folk are pushed out of the labour market. But most of those who come from the Accession countries are young, often highly educated and - crucially - single and child-free.
"In our last wave of migration most people came, did the jobs no one else would, and rapidly brought their families to join them.
"The upside is that this is l argely dependent-free migration; but it is socially significant - an influx of young men or of young women will change any community.
"And they can compete for the better jobs with settled young workers."
Mr Phillips said that between July 2004 and March 2006, almost 5,700 Accession country nationals registered as bus, lorry and coach drivers.
There were more than 11,500 care workers, 1,400 teaching staff, 550 dental practitioners and over 1,750 GPs, hospital doctors, nurses and specialists.
He said the migrants arriving in Britain 100 years ago broke with the past, unlike those who can now keep the link alive through travel and communication.
"Indeed, we expect that the majority of those coming from the EU Accession countries will return home in a few years," he said.
Mr Phillips said the new immigrants were establish-ing their own places of worship, shops and even media outlets. "There's nothing wrong with these preferences," he said.
"But it does present the possibility that the range of areas in which we share experiences as a whole nation is shrinking daily."
He said rather than settling in large cities, new migrants were focusing on relatively small towns in large numbers.
Mr Phillips, who described Britain as "sleepwalking to segregation" last year, insisted immigrants were needed to sustain our workforce. But he went on: "In this new world of more rapid and more diverse immigration, coupled with an unprecedented threat to global security, we cannot continue to pretend that there are no costs faced by our changing communities."
Mr Phillips warned that Britain was verging on "unacceptably high" ethnic polarisation in some areas and claimed the education system was encouraging that division.