David Cameron is trying to lay a trap for Ed Miliband

In an interview with Buzzfeed UK, Prime Minister David Cameron attempted to ramp up pressure on Labour leader Ed Miliband to swear off a deal with the Scottish National Party (SNP).

Over recent days the Conservative Party has been calling on Labour to pledge that it would not do a deal with the Scottish nationalists to form a government if it fails to secure a majority in May's election. On current polls, however, doing so would effectively prevent Labour from being able to form a functioning government.

He told Buzzfeed: "I think it is despicable for Labour not to say that "we're not dealing with them [the SNP]"."

Election Forecast UK projects that Labour could win around 274 seats in May, with the Conservatives on 286. Even with the addition of the Liberal Democrats 25 projected seats neither party would be able to cross the 326 seat-line to form a majority two-party coalition.

Were these forecasts to be reflected in May's result both parties would have to find a third party with which to do a deal. For Labour, the obvious candidate would be the SNP (projected to win a staggering 42 seats, which would see it become Britain's third largest party).

Earlier on Monday, Miliband ruled out a formal coalition with the nationalists saying that "there will be no SNP ministers in any government I lead". However, the Conservatives were quick to point out that his phrasing leaves open the possibility that Labour could do some kind of deal to secure SNP votes, even if the party is not represented in government.

Cameron is gambling that by pressuring Labour to either swear off a deal with the SNP or publicly refuse to do so he can undermine the party's ability to reach a deal with the SNP to guarantee nationalist votes, or isolate the party from pro-union Labour voters.