Bulgarians and Romanians could face £1,000 on-the-spot fines if they are caught working illegally in Britain once the two Balkan countries join the European Union in January.

The fines - and penalties of up to £5,000 for businesses who employ them illegally - were announced today by John Reid, the Home Secretary, as part of a complex package of measures to prevent a flood of migrants after the next round of EU enlargement.

More than half a million Eastern European workers, mostly from Poland, have arrived in Britain since 2004, when eight countries joined the trading bloc, but government and local authorities fear a further influx could prove politically damaging.

In a written statement to the House of Commons, Mr Reid said that while a limited number of unskilled workers will be taken on to carry out jobs like fruit-picking, Britain would not continue with its "open-door" policy regarding work.

In a move reminiscent of Labour's on-the-spot fines for yobs, Romanians and Bulgarians - known as A2 nationals - who are caught working illegally face instant fines of up to £1,000. Under the rules a worker would face a further fine of £5,000 if their case goes to court and they are convicted.

Mr Reid said: "Employing illegal workers undercuts legitimate business and leads to exploitation. It will not be tolerated."

The new restrictions are in stark contrast to the treatment of Poland and seven other nations which joined the EU two years ago.

Britain was one of only three EU member states to allow their citizens unrestricted access and the Government predicted that up to 13,000 would come to Britain each year from those countries.

So far, according to official estimates, as many as 600,000 have arrived, pushing down market rates for many manual jobs.

Doubts were immediately raised over the ability of the Home Office to enforce the complicated restrictions, in light of the department's previous failures on immigration issues. Charles Clarke, Mr Reid's predecessor, was sacked as Home Secretary in May after it emerged that more than 1,000 foreign prisoners had not been considered for deportation after serving their sentences.

Keith Vaz, the former Labour Europe Minister, said: "These restrictions will be unworkable, undesirable and unnecessary, and of lasting damage to the reputation of the UK as champion of the enlargement.

"There is no evidence that welcoming migrants from Eastern Europe in 2004 has had anything otherwise than beneficial effects for the United Kingdom.

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