More than three million migrants came to Britain under the previous Labour government, campaigners claimed today.
Migration Watch UK said official figures to be released on Thursday will show for the first time that net migration since Labour came to power in 1997 topped the three million mark.
Sir Andrew Green, the think-tank's chairman, said: "The sheer scale of what has occurred is changing Britain fundamentally and irrevocably and in ways the majority of the population did not ask for, were not consulted about and did not wish to see."
Describing immigration policy as "Labour's great betrayal", Sir Andrew said the introduction of 3.2 million people - three times the population of Birmingham - was increasing the pressure on "roads, railways, housing, infrastructure, the environment, schools, hospitals and the general quality of life".
He added: "The present Government will have to stick to their guns if they are to clear up the shambles they inherited and get a grip of developments that could otherwise fundamentally change the whole nature of our society."
Immigration Minister Damian Green said: "Unlimited migration has placed unacceptable pressure on our public services over the years.
"That is why we are currently carrying out major reform of the system to reduce net migration to the tens of thousands.
"We have already introduced an annual limit to the number of economic visas from outside the UK alongside new proposals to reform other routes of entry including student, marriage and settlement visas which have in the past been subject to widespread abuse."
Shadow immigration minister Gerry Sutcliffe said the Migration Watch report was "unbalanced, misleading and highly political".
"Migration levels increased initially because of the strength of the British economy over many years and must be seen in the context of increases globally," he said.
"However, the most recent figures show net migration from outside the EU was coming down as a result of the new points-based system and over a third of 'long-term migrants' were, in fact, students, the vast majority of whom study, pay their fees, and then return home.
"Labour introduced a tough Australian-style points-based system to ensure that only those with the skills Britain needs could come here.
"We established a single border force to strengthen our borders, 100% biometric visas and set up the Migration Impact Fund, paid for by contributions from migrants, to help local areas and services.
"The Tory-led Government's plan for a cap on migration lacks substance and credibility.
"The cap only applies to 20% of non-EU migration, and doesn't include students. And the cuts of over 5,000 staff from the borders agency will not make borders more secure.
"People are concerned about the impact of migration - that's why controls need to be strong, effective and fair."