LONDON (Reuters) - The success of British Prime Minister David Cameron's flagship immigration policy was called into serious question on Sunday, dealing him a potential blow ahead of 2015 elections by playing into the hands of a populist anti-immigration party.
A report by lawmakers from across the political spectrum concluded that Britain's migration statistics were grossly unreliable and "not fit" for purpose, undermining the credibility of Cameron's assertions about the issue.
Immigration is a hot button issue in the country, where Cameron is trying to stop an exodus of voters to the Independence Party (UKIP) before a parliamentary election in two years' time, and polls show the issue is one that worries voters the most.
Concerns have been fuelled by warnings in the right-leaning press about "hordes" of Romanians and Bulgarians moving to Britain next year when EU freedom of movement restrictions lapse, at a time when Britons face rising competition for jobs.
To compound Cameron's problems on immigration, a government pilot scheme that involves vans driving around London with billboards telling illegal immigrants to "go home or face arrest" has angered his junior coalition partner, the Liberal Democrats, and prompted allegations of racism.
Questioning immigration was for years regarded as racist, but all three main parties have started to talk tough about the subject in response to rising public anxiety, campaigns in the country's popular tabloid press, and the popularity of the anti-immigration UKIP.
Cameron's ruling Conservatives, who are trailing the opposition Labour party in the polls, have promised a far-reaching immigration crackdown and say they have cut net migration - the numerical difference between people coming in and out of the country - by a third.
They have also pledged to cut that figure to tens of thousands of people a year from over 100,000 now.
DATA 'TOO UNCERTAIN'
But the report, by the lower house of parliament's Public Administration Committee, said that government migration data was neither systematically nor rigorously collected.
"We are struck by the advice of the Migration Advisory Committee to the Government that it should aim for net migration of only 50,000 as the only means of being certain that net migration is in fact below 100,000," it said.
Bernard Jenkin, a Conservative lawmaker who oversaw the report, told local media government statistics were "little better than a best guess and could be out by tens of thousands."
Chris Bryant, a Labour lawmaker, said the report showed the government's assertions on immigration could not be trusted.
"We should be able to count people in and out of this country," he told Sky TV. "If the government is going to boast about having cut net migration then you would think that the statistics would be reliable ones. The truth is they're not."
Cameron's Conservatives are trailing Labour in the polls, but several surveys show Labour's lead, which until recently had been fairly steady at ten percentage points, is narrowing.
One opinion poll published earlier this month put the Conservatives in joint first place with Labour for the first time in over a year.
UKIP, which campaigns for Britain to leave the European Union and for a halt to "open door immigration," made sweeping gains in May local elections, winning almost one in four votes.
But its poll rating has since fallen from a high of 22 percent. Some polls suggest it has lost one or two percentage points. Others suggest its fall in support may be more serious.
A spokesman for Britain's Home Office (interior ministry) said the government was confident it was on the right track.
"We disagree with the report's conclusions," he said. "Government reforms on immigration are working and the statistics do show that net migration is at its lowest level for a decade."
(Editing by Mike Collett-White)