By Kylie MacLellan

LONDON (Reuters) - Britain's border and immigration system is a "mess", lawmakers said on Wednesday in a report which will increase pressure on Prime Minister David Cameron's government to take a tougher line on migrants ahead of next year's national election.

Opinion polls show immigration is a major concern for voters and it has fueled the rise of the UK Independence Party, which may scupper Cameron's chances of staying in power after the May election by splitting the right-wing vote. UKIP backs stronger immigration curbs and Britain's exit from the European Union.

In a damning report, parliament's Public Accounts Committee said the government could not track people through the system or check whether those refused the right to stay in Britain had actually left the country.

The government has also failed to tackle a long-standing backlog of tens of thousands of asylum applications and the number of new cases awaiting a decision is increasing, it said.

"The pressure is on and the Home Office (interior ministry) must take urgent steps to sort out this immigration mess," said opposition Labour party lawmaker Margaret Hodge, chair of the committee.

At the end of the 2013-14 financial year there were more than 175,000 people whose applications to stay in Britain had been rejected, the report said.

"It is deeply worrying that the Home Office is not tracking those people ... to ensure that they are removed from the UK," said Hodge. "The department should, as a matter of urgency, take more steps to identify people who remain in the UK illegally and speed up their removal."

A YouGov poll published on Wednesday showed 57 percent of voters considered immigration the most important issue facing the country, with the economy coming second on 49 percent.

The political debate in Britain around immigration has become increasingly heated as Cameron and other party leaders have hardened their stances on the issue in response to the rise in popularity of UKIP.

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LOSING CONTROL

Nick Boles, a government minister and member of Cameron's Conservatives, said Britons felt parliament had lost control over immigration and might never regain control due to EU rules on free movement within the 28-nation bloc.

"The difficulty that has arisen is this sense that we don't have that control and, bluntly, they're right. It's true," he said in an interview with Total Politics magazine.

Defence Secretary Michael Fallon spoke on Sunday of some British towns being "swamped" by migrants. On Monday he apologized for his use of emotive language but said the thrust of his remarks about immigration was correct.

The mayor of the northern French port town of Calais, from where migrants often attempt to cross the English Channel to Britain, urged London on Tuesday to overhaul its generous welfare system and lax identity controls, saying they made it a magnet for illegal immigrants.

The new report said outsourcing group Capita had been hired in 2012 to check the records of more than 250,000 people who should have been removed from Britain and by the end of last year it was unable to trace more than 50,000 of them.

The committee, made up of lawmakers from Labour, the Conservatives and their junior coalition partner the Liberal Democrats, said it was "disturbing" that the government did not know where these people were.

(Editing by Gareth Jones)