Cameron: 'A Vote For UKIP Is A Vote For Labour'

David Cameron has launched a fresh effort to try to persuade would-be UKIP voters to support the Conservatives, warning it would be a "terrible irony" if voting for Nigel Farage's party led to Ed Miliband becoming Prime Minister.

Mr Cameron said voters should not be "deceived" into thinking the next election was anything other than a "stark choice" between the Tories and Labour.

The PM repeated his promise to put measures to control EU migration at the heart of his plans to renegotiate the UK's relationship with Brussels.

Reports suggest Mr Cameron could bring in a cap on National Insurance numbers issued to low-skilled migrants from EU countries in a bid to cut immigration from the 28-member bloc.

The Sunday Times reported that this would see new arrivals being given a National Insurance number for a temporary period, preventing them from working and claiming tax credits in the UK indefinitely.

But the outgoing president of the European Commission, Jose Manuel Barroso, warned an arbitrary cap would not be compatible with European legislation.

Speaking on The Andrew Marr Show, he said any cap on free movement would be "in principle" a contradiction of EU rules.

"It is very important to have a positive tone regarding these issues between Britain and the EU," he said.

"My advice to all the members ... is to have a constructive dialogue, a proper tone."

Former deputy prime minister Lord Heseltine told Sky's Murnaghan programme it was a "perfectly legitimate" subject for Mr Cameron to raise.

The general election in May will be "the most important for a generation", the PM said in an article for the Sunday Telegraph.

He said it would be a straight contest between himself and Mr Miliband for the keys to Downing Street.

In a message to wavering Tories, he said: "Let no-one deceive you that there is a third way. A vote for UKIP is a vote for Labour.

"That much was proved in the recent by-elections. In Clacton, previously a Conservative seat, UKIP won. In Heywood and Middleton, Labour held their seat.

"And to those considering voting for UKIP on the issues of immigration and Europe, I would point you to our Conservative record."

The PM said this included his veto of an EU treaty, a pledge to scrap the Human Rights Act and an in-out referendum in 2017.

"There would be a terrible irony if people who care about these issues voted UKIP - making a Labour Government more likely."

The PM warned letting Mr Miliband into Number 10 would lead to a "great nation slipping back into decline", with households facing rising mortgage bills and jobs going overseas.

Shadow immigration minister David Hanson said: "Labour is in favour of reform to European free movement rules and we will examine any proposals the Government comes forward with to manage immigration with interest.

"But why should anyone believe the Prime Minister when he has a record of making big promises on immigration and not delivering, when everyone knows he wants headlines for the Rochester and Strood by-election, and when the briefing from the Government keeps changing every couple of days?

"The Prime Minister has made pledges on immigration before, promising that he would get net migration down to the 'tens of thousands', 'no ifs, no buts'. But he had no practical policies to deliver it, and that target is now in tatters."