Too many migrants working in the UK are being exploited by bad employers, a leading trade unionist has said.
Migrant workers particularly work in construction
Jack Dromey, of the TGWU, was reacting to a new EC report saying the UK has benefited from opening up its markets to workers from central Europe.
Since May 2004, 293,000 workers have applied to be registered to work in the UK. The government originally predicted there would be only 13,000 applicants.
The Migrationwatch UK group says this causes problems for low paid workers.
The European Commission report says new workers from states which joined the EU in 2004 have boosted growth without increasing unemployment.
Most older EU member states restricted access to their labour markets.
But the UK, Ireland and Sweden opened their labour markets and the report suggests they have reaped benefits.
Mr Dromey, deputy general secretary of the TGWU, said the British economy needed migrants from all over the world.
"The building cleaners, those who work on building sites in London, they are welcome," he told BBC News 24.
"What is wrong is that too many of them are being exploited by bad employers. That must end. Employers have a responsibility and government has a responsibility."
The Joint Council for the Welfare of Immigrants urged the UK to sign an international agreement to protect the rights of migrant workers.
And migrant workers in Britain without documents should get some permission to remain in the country if they reported employment abuses, said the group.
The council's legal and policy director, Tauhid Pasha, said: "The reality on the ground which the government is failing to face up to are trafficked and undocumented workforces in the UK who are currently excluded from any protection by way of their immigration status."
The government's own figures suggested there were up to 570,000 illegal immigrants, she said.
They were more attractive to "exploitative employers" because they had no enforceable rights at work.
'Impact on wages'
The EC report says the flow of migrant workers has not been big enough to swamp labour markets.
The new workers fill gaps in labour markets, particularly in construction and catering, it says.
Sir Andrew Green, from the Migrationwatch UK, which warns about migrant levels, said: "This is good news for employers who get cheap labour but it is bad news for the low paid, whose wages are being held down.
"It also makes it much more difficult to move from welfare to work for the 4.2m people who are on incapacity benefit or are unemployed."
Sir Andrew said it was nonsense to suggest migrants only filled vacancies.
Migrants both filled vacancies and created demand, so arguing the case from the existence of job vacancies was to argue for an "endless cycle of immigration", he said.
Earlier this year Home Office Minister Tony McNulty admitted the government was "in the dark" when it estimated how many workers would come to the UK after the EU expanded in 2004.
Mr McNulty said many of the 293,000 applicants for the Workers' Registration Scheme had not stayed in the UK.
Estimates were also skewed as other EU states had not yet let workers in, he said.