(LONDON) - One million immigrants came to Britain after the European Union's 2004 expansion but half of them have now gone home, a study said Wednesday, reporting a reversal of the massive influx of workers.

Growing prosperity in EU newcomer states is drawing migrants home, while the falling value of the pound is also making working in Britain less attractive, said the Institute for Public Policy Research (IPPR).

"Four in 10 of the returned Polish migrants we surveyed think that better employment prospects in Poland would encourage Poles living in the UK to return to Poland for good," said the centre-left thinktank.

The huge wave of immigrants from countries like Poland, Lithuania and Slovakia took British authorities by surprise after the EU took in 10 mostly ex-communist countries four years ago.

The government initially forecast only that thousands of workers would take advantage of Britain's open-door policy to earn relatively large salaries here. In the event huge communities rapidly formed in London and across the country.

The IPPR study estimated that 665,000 nationals of the 10 EU newcomer states were living in Britain in the last quarter of 2007 -- a massive increase of 548,000 since the first quarter of 2004.

The new immigrants -- whose arrival has been compared in scale to that of south Asian and West Indian influxes from former British colonies in the 20th century -- are spread extensively round the country.

As well as London they have notably moved to Scotland and southwest England, which do not traditionally attract newcomers.

The 10 countries which joined the EU in 2004 were: Poland, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Slovakia, Slovenia, Cyprus and Malta. Another two, Bulgaria and Romania, acceded in 2007.

Britain was one of only three EU states, along with Ireland and Sweden, which immediately opened their doors to workers from the newcomer states. The rest imposed transition periods on the new arrivals.

London, though, imposed caps on the numbers of Bulgarians and Romanians, restricting entry to highly-skilled and seasonal workers.

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