Britain's immigration chief has refused to speculate on how many Bulgarians and Romanians will come to the UK when the two countries join the EU next month.
Romanians may face work restrictions
Immigration minister Liam Byrne said: "It would be very unwise to make predictions about flows."
New EU migrants will face work curbs after concerns about the potential impact on local schools, hospitals and housing, Mr Byrne told MPs.
But all Romanians and Bulgarians will be free to live in the UK, he said.
Asked by Conservative MP David Heathcoat-Amory how this would prevent people coming to the UK to work in the "black economy", Mr Byrne said the restrictions on working were not an attempt to limit immigration.
"It's an error to say that we are trying to manage migration in this way, ultimately people from Romania and Bulgaria do have the right of free movement into this country and that's just a deal we signed up to when we signed up to the accession treaty.
"What we can do is exercise our derogation on controlling access to the labour market. That does have knock-on implications for access to benefits."
Home Secretary John Reid recently unveiled a package of penalties to enforce a 20,000 cap on the number of migrant workers from Romania and Bulgaria.
But, appearing before a joint session of the Commons Home Affairs and European Scrutiny Committees, Mr Byrne said: "While there is the possibility that people can come and work self-employed, I think there is the potential for abuse."
He said Britain's policy was not to close the door to workers from the two countries "forever" but for an initial 12 month period to fully assess the impact of the first wave of new EU migrants.
He also wanted to make sure there were stronger safeguards in place to prevent illegal working - something he said would happen over the next 12 months.
He insisted it had not been a mistake to admit workers from former Eastern Bloc countries, arguing it had had a positive impact on the British economy, particularly in reducing inflation.
But he told the committee: "There has been anecdotal information of specific and isolated pressures.
"So, for example, some local schools we know have struggled to cope, we know some local authorities in different parts of the country have reported pressures typically of private housing - while that evidence exists I think it is incumbent on the government to understand it in a bit better detail."
He said he wanted local councils to be given better and more rapid access to information on migration flows to help them plan services.
But he also told the committee he could not say how many migrants would come to Britain from Bulgaria and Romania.
In 2003, research commissioned by the home office predicted 13,000 migrants workers would come to the UK after eight Eastern European countries joined the EU.
In the event about 600,000 migrants have arrived so far, mainly from Poland.
Committee chairman John Denham said: "The position today seems to be that the home office has decided that given that experience we would rather not ask the question at all when it comes to Bulgaria and Romania.
"Because it would seem nobody has even attempted to estimate what might happen or what the implication of your new policy is."
Mr Byrne replied: "I think the committee would draw its own conclusions about the value for money that the home office obtained when it commissioned this research and perhaps even the National Audit Office would be interested."
He said ministers had been fully aware of the potential weaknesses in the research and the decision to open the door to Eastern European workers had been a "political" one.
"Very often political decisions are made without perfect information," Mr Byrne told the committee.
"The choice that we had to make here is not 'do we stop migration from Bulgaria and Romania forever?', it was simply a question of how quickly we open access to our labour market."
He said it was right to take a gradual approach until impacts could be "fully understood".
Labour MP Ann Cryer asked if the new curbs "may just increase the number of people who are bogus self-employed migrants or who are undeclared workers".
Mr Byrne said there were always "risks" of abuse with any "restrictive regime".
But he said the UK home office had been working with the Bulgarian and Romanian governments on tackling organised crime gangs, which were mainly behind illegal immigration.
"When you have got a situation where pretty much all the big European economies are making the same decisions as us, that gives me cause for comfort," he added.