A new survey suggests there could be as many as 100,000 migrant workers employed on building sites in the UK.
As many as one in 10 building site workers could be migrants
One Polish carpenter tells the BBC how he went from working on the black market to becoming a legitimate construction worker.
Lukas came to the UK from Poland six years ago and now lives in east London with his wife and young son.
He told BBC Radio Four's Today programme: "The wages are higher, the daily rates are higher. It's more expensive to live here but you can still make a good living."
It's more expensive to live here but you can still make a good living
Lukas used to work on the black market but, tired of poor pay and tired of looking over his shoulder all the time, he decided to enter the mainstream building trade.
The first step was to get a temporary CIS4 card - or Construction Industry Scheme Four - an identity document issued by the Inland Revenue.
It means his employer deducts 18% tax at source.
"That was relatively straightforward. I think it was proof of address, passport and a form my solicitor gave to me and filled it in for me. That was all.
"I think after a week I received my temporary card."
No immigration check
According the Inland Revenue, anyone can get a temporary CIS4 card, which are valid for one year and then renewable.
There is no check on immigration status and nationality is not recorded.
But it does not provide proof of right to work.
Only a permanent CIS4 card with a permanent National Insurance number does that.
However, there is widespread confusion within the building industry and a CIS4 card is all that is required on many building sites, including those run by the biggest construction firms.
They generally do not employ workers directly, using sub-contractors instead.
Temporary CIS4 cards are valid for a year and are renewable.
It is not hard, though, for skilled tradesman like Lukas to legitimatise their status.
If they are from the former communist states joining the EU in May, or if they are from Romania or Bulgaria, they can get a business visa as self-employed if they convince immigration officers they have a valid business plan.
That, in turn, will give them a National Insurance number.
Lukas has followed that process all the way through.
He said: "It was quite difficult. The first visa I got was for one year and then, after I got my National Insurance number and a permanent card, I had to provide some other papers.
"I got a visa for another three years. I had to prove I was self-employed by showing receipts, bank statements and invoices."
Lukas now has a permanent CIS4 card, one of 560,000 in use in the UK.
I think it's enough if someone's willing to pay taxes and can get work here - he doesn't need to apply for any benefits, does he?
There are many eastern Europeans who are still in the black sector of construction, working without any kind of CIS card for low wages, paying off agents who helped them get work and often frightened the immigration officers will find them.
Many in the industry hope such practices will die out after 1 May because it should be far easier for citizens of EU accession states to work legitimately, even if they are here already.
But those like Lukas say welfare benefits should not be extended to people like him. His own visa does not allow him to claim them.
He said: "To be honest, I don't think we should be entitled to all the benefits British people are entitled to because a lot of people will take try and take advantage of the system - that will reflect on all of us.
"I think it's enough if someone's willing to pay taxes and can get work here - he doesn't need to apply for any benefits, does he?"