Dutch social affairs minister Lodewijk Asscher has called on the EU to focus on the "negative consequences" of labour migration from Romania and Bulgaria at a time when new studies show the fear of massive influxes to be overblown.

"In the Netherlands, a 'Code Orange' is issued as the water in the rivers reach an alarmingly high level. It is now time for a similar alarm, namely about the sometimes negative consequences of the free movement of persons within the EU," Asscher wrote in an op-ed for De Volkskrant newspaper on Saturday (17 August).

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Labour migration from Romania and Bulgaria has become a hot topic in Britain, Netherlands and Germany before the last remaining labour market restrictions will be lifted on 1 January.

Co-signed with David Goodhart, a British campaigner for restrictions on immigration, the Dutch op-ed highlights wage dumping, exploitation and "crowding out" of local workers as main "negative side-effects" to freedom of movement of EU workers.

The two call for stricter rules punishing companies who exploit labourers from the new member states.

The EU law allowing freedom of movement for self-employed workers came into force in 2006 and was initially tabled by a Dutch politician, Frits Bolkestein, who at the time was EU commissioner for internal market. Bolkestein was a member of the same Liberal party led by the current Dutch Prime Minister, Mark Rutte.

The law was harshly criticised at the time by trade unions across the EU who warned against a race to the bottom on wages and social benefits once poorer eastern Europeans - generically dubbed "the Polish plumber" were allowed to work freely in the richer western countries.

Fears of mass labour migration from Poland and other eastern countries proved to be overblown, however.

The same goes for Romania and Bulgaria, argues Migration Matters, a UK-based lobby group that says numbers floated in Britain are 10 times more than what is realistically to be expected.

"Our belief is that the anti-immigration lobby [in the UK] have cried wolf once too often," Atul Hatwal, director of Migration Matters, told The Guardian over the weekend.

"Their claim is that as many as 300,000 new migrants will arrive from Romania and Bulgaria over the course of 2014. In truth we believe that figure will probably peak at around 20,000," he added.

Last week, the British Office for National Statistics estimated that 37,000 extra Romanian and Bulgarian workers have arrived since June 2012.