More than three quarters of a million immigrants came to Britain in 2005 — far more than official figures admit, according to a report published yesterday.

The numbers helped to send net migration soaring by an estimated 400,000 rather than the government figure of 185,000.

The report by the City forecaster Capital Economics also suggests that 50,000 workers from Romania and Bulgaria will head to Britain this year.

Although the Government has placed restrictions on low-skilled Bulgarians and Romanians coming to work here, they can still come as visitors. “It is quite likely that many will stay and work in the black economy,” the report said.

The report also said that the arrival of more than 600,000 migrants from eight former Soviet bloc states since 2004 would continue to encourage others to travel to seek work. In the short term this would lead to the workforce growing more rapidly than employment over the next two years, with a consequent rise in unemployment.

Ministers imposed curbs on low-skilled workers from Bulgaria and Romania in response to concern among Labour MPs that a growing migrant workforce was leading to unemployment among British citizens and putting pressure on housing and schools.

The Capital Economics report said that it was unwise to rely on official figures which suggested net migration had peaked and had fallen from 223,000 in 2004 to 185,000 in 2005. The International Passenger Survey, on which official figures are based, was unreliable as it only counts people saying they intend to stay for more than a year, it said. When workers from the eight former Soviet bloc states were included in figures, the overall level of immigration rose to 780,000 in 2005 compared with a government figure of 524,000.

The report estimated that net migration — the difference between those arriving and those leaving — rose to 400,000 in 2005 compared with a government figure of 185,000.

Despite other states in the European Union lifting employment restrictions for citizens from the eight states, Britain would remain an attractive destination. “We doubt there will be a large shift of immigration from the UK to the countries which have opened their borders. After all, for many immigrants, English will be their preferred second language,” the report said.

But it added that although Britain would remain attractive to East Europeans, net migration might slow: many East Europeans would return home after about two years.

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