The influx would equal a city the size of Leicester.

A new analysis estimates 50,000 Romanians and Bulgarians could arrive each year — or 250,000 over the next five years. But that figure could hit 70,000 annually — or 350,000 over five years, according to campaign group Migration Watch.

Chairman Sir Andrew Green said: “It’s likely to have significant consequences for housing and public services. It will also add to the competition which young British workers already face.”

The statistics come just days after Communities Secretary Eric Pickles refused to publish the Government's estimates on immigration from Romania and Bulgaria.

He said he was “not confident” they were accurate. The Home Office last night insisted ministers were working to cut overall migration.

A spokesman said: “We’ll apply transitional controls on new EU member states.”