More than 100,000 Romanians have applied for the right to work in Britain since the autumn of 2013.
The count of Romanians who asked for National Insurance numbers – necessary for anyone who wants to work legally in Britain – has risen nearly fivefold in a year.
The explosion in numbers of Romanians came alongside a sharp rise in applications from Bulgarians for National Insurance documents.
Queue: Romanians living in Portsmouth line up to cast their vote in elections in their home country earlier this month
Presence in Britain: Romanians gather outside their embassy in London two weeks ago to protest that they were unable to vote in the election
They appear to include a large number of people who were already in Britain before Romanians and Bulgarians were given the right to work in January.
Sarah Crofts, of the Office for National Statistics, said that half of those applying for National Insurance numbers were ‘people who had been in the UK for a year or longer’.
The ONS analysis raises the possibility that many of the Romanians and Bulgarians may have been working illegally in Britain in the black economy.
ONS immigration figures said immigrants in the year to June 2012 included 32,000 people from Romania and Bulgaria. Of these arrivals, 24,000 are estimated to have come to Britain to work.
The 103,900 National Insurance numbers issued to Romanian citizens in the year to September represented a 468 per cent rise on the 18,300 applications recorded in the previous 12 months.
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