Restrictions on Romanians and Bulgarians working in Britain could be "too draconian" and may have to be eased within 12 months, the Government has acknowledged.
A Home Office minister had faced accusations that the tough stand had damaged relations with the two east European countries, which join the European Union on 1 January.
After that date, lower-skilled Romanians and Bulgarians will be allowed to take up only farming and food-processing jobs, and their numbers will be capped at fewer than 20,000 a year.
Liam Byrne, the Immigration minister, was challenged in the Commons over a letter to The Independent this week from Richard Thomas, a former British ambassador to Bulgaria. Mr Thomas said that ministers had "behaved despicably" by treating the two new member states as "second-class citizens" and said he would be ashamed to show his face again in Bulgaria.
Wayne David, the Labour MP for Caerphilly, said the comments were a "damning indictment" of government policy and warned "irreparable damage" could have been done to Britain's relationships with both countries.
And in a submission to yesterday's joint meeting of the Commons Home Affairs and European Scrutiny Committees, the Romanian embassy in London protested at the "discriminatory approach" towards its citizens.
Mr Byrne told the session that other major west European nations including France and Germany had placed similar restrictions on Romanian and Bulgarian workers. But he said there would be an early review of the curbs, adding: "We were aware that the restrictions we imposed may be too draconian."
The restrictions could run for five years, but they will be reviewed in a year to ascertain their impact on the UK economy and jobs market.
Mr Byrne refused to be drawn on how many economic migrants could come from the two countries. After the massive underestimates of the impact of the previous EU expansion, he said it would be "very unwise to predict future flows".
In a stinging rebuke, the former Home Office minister, John Denham, protested: "The Home Office would rather not ask the question at all in relation to Romania and Bulgaria. It would seem that nobody has made any assessment of what might happen."
Mr Byrne said the previous EU expansion had boosted the British economy, but he acknowledged there was anecdotal evidence that it had caused "isolated pressures" on public services.
"Some schools have struggled to cope through an influx of children. We know that some local authorities have reported pressures of overcrowding, particularly over private housing."
He also said that the rule allowing Romanians and Bulgarians to be self-employed in Britain meant there was "potential for abuse" which would be combated by "robust" enforcement measures.
Damian Green, the shadow immigration minister, said last night: "It is not good enough for the Government not even to make a public guess as to how many migrants will come here simply because they got the figure so wrong last time.
"If they really have no idea, it is likely that some parts of the country will feel pressures on their public sector infrastructure, similar to those felt by towns such as Slough following the last wave of EU accession."