LONDON -- When Poland and seven other formerly Communist nations joined the European Union in 2004, Britain threw open its jobs market earlier than required to welcome tens, even hundreds, of thousands of new workers born behind the Iron Curtain.
A decade later, the prospect of Romanians and Bulgarians' being able to work freely here beginning next year has provoked protests in Parliament and the press. In a sharp illustration of how the immigration debate and Britain's economic fortunes have shifted, the government was forced to deny that it planned a negative advertising campaign in Romania and Bulgaria to discourage people from coming to Britain.
Prime Minister David Cameron says Britain should not be a "soft touch" for migrants and his ministers are discussing ways to limit access to benefits or public services for those arriving from abroad. In order not to break E.U. rules against discrimination, there are likely to be changes for Britons too, and one newspaper reported Monday that plans were being drawn up to introduce an "entitlement card" that anyone wanting access to services would need to produce.
Among Mr. Cameron's Conservatives, pressure for action is growing and is only likely to increase after a by-election last week in which the party was pushed into third place by the U.K. Independence Party, which opposes mass immigration.
Even before that shock result, there was a warning from Eric Pickles, Secretary of State for Communities and Local Government, that "any influx from Romania and Bulgaria is going to cause problems."
"My constituents," added Philip Hollobone, a Conservative lawmaker, "think it is madness to open our borders to 29 million people when we have absolutely no idea how many are going to come to this country."
Back in 2004, a healthy British economy seemed to benefit from globalization, booming financial services and open markets. Politicians saw immigrants giving a jolt to growth.
Now a stagnant economy may slip into a triple-dip recession and Britain is experiencing another of its regular bouts of anxiety about immigrants.
"Immigration has always been a hot issue," said Sandra McNally, professor of economics at the University of Surrey, whose research concluded that immigration from Eastern Europe after 2004 had, if anything, a positive impact on educational standards in Britain. "But it has become a lot hotter."
The discourse is particularly febrile because, before 2004, a study commissioned by the government hugely underestimated net immigration from Eastern Europe, suggesting 5,000 to 13,000 arrivals a year up to 2010.
In fact, the 2011 census showed 521,000 Polish-born people listed as residents in Britain, with the vast majority having arrived after 2004.
Back then, most other E.U. countries exercised their right to use "transitional arrangements" to restrict access to the jobs market for the eight new ex-Communist members, including Poland, for a maximum of seven years.
But along with Ireland and Sweden, Britain opened up from day one.
By 2007, when Romania and Bulgaria joined the bloc, it was clear that many more immigrants had arrived in Britain from Eastern Europe than expected. So this time the government decided to apply transitional restrictions on Romanians and Bulgarians. Those measures expire on Dec. 31 in all the E.U. nations where they remain, including Britain.
Last year, Ed Miliband, now leader of the Labour Party, conceded that the Labour government got it wrong in 2004 by opening up the jobs market too fast, exposing lower-paid workers to extra competition.
"We were dazzled by globalization and too sanguine about its price," Mr. Miliband said. "By focusing exclusively on immigration's impact on growth, we lost sight of who was benefiting from that growth -- whose living standards were being squeezed."
The case for Romania and Bulgaria is hardly helped by their ranking among Europe's poorest nations, both struggling to combat corruption and organized crime, and to integrate Roma populations.
Migration Watch, a non-governmental organization that campaigns on immigration issues, suggests that inflows could average around 50,000 annually over the next five years. Tabloid newspapers have taken up the issue and The Daily Mail has warned, for example, of "mafia bosses who can't wait to flood Britain with beggars."
Nevertheless, because of E.U. law, the government's options to act now are limited. "Restrictions on workers from new member states like Romania and Bulgaria cannot be extended unilaterally," said Jonathan Todd, spokesman for the European Commission on employment and social affairs.
Already, Britain faces legal action from the commission for interpreting residency test rules too strictly, Mr. Todd said.
The population flows, however, may not be as significant as many expect.
Contrary to what many Britons believe, its borders have been open since 2007 when Bulgarians and Romanians were allowed to enter Britain like other E.U. citizens. The difference is that they have no automatic right to take most jobs and, apart from a handful of job categories, need to secure a permit to work.
According to Bulgaria's ambassador to Britain, Konstantin Dimitrov, around 53,000 Bulgarians work legally in the country, which suggests that many of those who want to come have already done so. While professing surprise at the tone of the current debate in Britain, he added philosophically that the "economic crisis predisposes people to seek an external whipping boy."
Figures from the European Commission also suggest much of the migration has already taken place. By the end of 2010, around 2.9 million Bulgarian and Romanian citizens lived in other E.U. nations, more than twice as many as shortly before those countries joined the Union.
But the two main destination countries were Italy and Spain, which together housed more than 70 percent of all Bulgarian and Romanian nationals resident in another member state.
The depth of recessions in Spain, Italy and Greece could push people to Britain. Yet next year Romanians and Bulgarians will have more countries to choose from than Poles did in 2004. And Germany's relative proximity and better economy could make it a more likely destination than today's Britain.
This article originally appeared in The New York Times.