It will not be possible for Britain to restrict the freedom of movement of Bulgarians and Romanians when transitional controls are lifted, the European Commission's vice-president has warned.
Prime minister David Cameron told ministers earlier this week to prepare action to ensure that the UK's public services and welfare system are not a "soft touch" for migrants, including those coming from other EU states.
Cameron chaired a Whitehall committee on the issue on Tuesday, amid fears that the lifting of transitional controls at the end of this year may spark a wave of immigrants from the EU's newest member states.
But EU Justice Commissioner and Commission vice-president Viviane Reding said on Thursday that from 2014, Romanian and Bulgarian nationals must be treated in the same way as citizens of other member states, who have the right to work and settle in the UK.
"It is necessary to have the same rights for the citizens all over Europe," Reding told BBC Radio 4's World at One.
"Britain was always one of those member states who pushed for the enlargement of Europe and that is why we are now building minimum standards in this enlarged Europe in order to have an equilibrium of rights for the citizens - all citizens.
"There are one million British citizens living in another member state. Do you think those member states can discriminate against them because they are British citizens? You treat them the same as you treat national citizens.
"The free movement directive is not a directive on which Britain can opt out. Free movement is one of the foundations of the single market and the single market is something which the UK is very attached to and it is one of the most cherished EU rights.
"It is a two-way street - it has rights and obligations - but what you can't do is discriminate."
The PM's official spokesman told reporters today that the government was not planning to publish predictions of the number of incomers expected from Romania and Bulgaria, because of the "very inaccurate levels of inaccuracy" in estimates of eastern European migrants made by the previous Labour administration in the last decade.
Instead, a study by the National Institute for Social and Economic Research into the possible impact of additional migrants will be published "in due course".
"[communities secretary Eric] Pickles says he has seen estimates of the number of people who will come from Bulgaria and Romania, but he doesn't believe them. [home secretary Theresa] May says she hasn't seen any estimates.
"Some say there are estimates but they refuse to publish them. Others say they haven't even asked the question.
"You couldn't make it up. As every week goes by people are coming to the sad conclusion that the government hasn't a clue.
This government should trust the people of Britain's judgment and publish the figures they do have."
Meanwhile, Reding also warned that Britain faces a lengthy and expensive process if it chooses to exercise its right to opt out of certain aspects of pan-European rules on crime and justice.
May has indicated that the government plans to exercise its opt-out on the 133 measures in 2014, before negotiating to opt back in to those which are deemed to be in the national interest.
But Reding said: "I think that would not be very wise... It is very, very complicated and complex, time-consuming, with legal uncertainty and costs."
Britain would have to opt out of all justice and home affairs matters before applying to opt back in to those which it wanted to share in, she said.
And she added: "There may be financial implications for GB because GB will have to pay for the damages which are done by this opt-out."
Reding made clear that Britain's ability to opt back in to selected measures would not be a foregone conclusion, as the Commission would first have to consider the "coherence" of the UK's demands and their impact on the overall architecture of EU security policies.