A Liberal Democrat Cabinet minister has accused senior Tories of "pandering" to anti-European sentiment putting thousands of jobs at risk.
Danny Alexander, the Treasury Chief Secretary, said an upsurge in opposition to the EU in next year's European elections would send a "shiver of doubt" through the boardrooms of companies considering investing in the UK.
Writing in The Independent, he said the Lib Dems would be the only party campaigning in the elections on an "unambiguously pro-European message".
His intervention comes after his Lib Dem colleague, Business Secretary Vince Cable, infuriated Conservatives by comparing their hardline approach on the lifting of restrictions on migrants from Romania and Bulgaria to Enoch Powell's "rivers of blood" speech.
Alexander said: "The fact that some senior Conservatives are arguing that Britain should vote to leave the EU is already unsettling investors and threatening jobs and growth. Further pandering to anti-Europeans would be bad for the British economy."
Alexander said the Lib Dems would campaign on the threat to British jobs if the country votes to leave the EU in David Cameron's promised referendum in 2017.
He cited research by academics at South Bank University which estimate 3.5 million jobs depend on exports to the EU.
"A surge in anti-European sentiment in the Euro elections would send a shiver of doubt into the boardrooms of global companies that locate in Britain because we are a gateway to the EU single market," he said.
"With Labour confused on Europe, and the Conservatives divided, only the Lib Dems will be campaigning at the European elections with an unambiguously pro-European message."
He added: "Pro-Europeans in Britain have been too quiet for too long. Next year is an opportunity to make our argument heard, and the Lib Dems will make sure we take it."
Cameron has also been attacked for for acting "irrationally and shortsightedly" over immigration from eastern Europe by Lech Walesa, the first elected president of post-communist Poland.
He told a Polish TV News programme: ""Poles finished communism and Great Britain profited significantly from this. [Cameron] should not forget this, he should do the maths. He should realise that Poles finished with this system at the cost of 70% of their economy. He should see this and then he will understand that Europe, that countries like Great Britain, are again behaving irrationally and shortsightedly."