ED Miliband has given up on seeking a majority Labour Government and wants to "crawl to power in Alex Salmond's pocket," David Cameron has insisted as the battle for No 10 intensifies.

In deeply personal exchanges at a raucous Commons question-time, the Prime Minister branded the Labour leader "weak and despicable" for refusing to rule out a post-election deal with the Nationalists, which, he said, would threaten the nation's defences because of the SNP's opposition to Trident "and put the country at risk".

He brandished a Scottish Labour leaflet, which urged voters to "stop the Tories being the largest party", arguing it showed Labour had now given up on seeking a majority and was campaigning simply to be the largest party itself at Westminster.

"They are not trying to win, they are just trying to crawl through the gates of Downing Street on the coat-tails of the SNP. So what he's got to do is prove he's not a chicken and rule that out," declared Mr Cameron to Tory cheers.

An animated Mr Miliband hit back, saying: "There's only one person preparing for defeat and it's this Prime Minister."

Conservative HQ also seized on remarks from Lord Mandelson, the former Labour Business Secretary, who indicated he did not expect Labour to win an outright majority on May 7, noting the election was almost certain to deliver a "stalemate hung Parliament".

Grant Shapps, the Conservative Chairman, said: "Lord Mandelson is confirming what we now all know; the only way Miliband will get into Downing Street is if he's carried there in Alex Salmond's pocket."

The political heat was raised at PMQs when the Labour leader once again challenged Mr Cameron to a head to head TV debate, accusing him of "chickening out".

Jabbing his finger, he told the PM: "Instead of these ridiculous tactics, why don't you show a bit more backbone and turn up for the head to head debate with me; any time, anywhere, any place?"

Mr Cameron replied: "I'll tell you what goes to character: someone who is prepared to crawl into Downing Street in alliance with people who want to break up the future of our country. What a despicable and weak thing to do: risking our defences, risking our country, risking our United Kingdom. If you had an ounce of courage you would rule it out."

Mr Cameron also sought to embarrass Mr Miliband, telling MPs: "He says we need the two leaders, we need the two leaders who can call the tune; that's me and Alex Salmond. So let's have the debate."

Tory HQ made clear the PM was not challenging the former SNP leader to a TV debate. A senior source said: "People need to take a deep breath and calm down. The Prime Minister was simply making the political point that if there were a pact between Labour and the SNP, it would be Mr Salmond calling the shots".

But the ex-party leader, who is seeking to return to the Commons by winning Gordon, responded to the PM's words, saying: "Cameron ran scared during the referendum campaign and now he's running scared again. He should have been called to account last year and should be called to account this year."

The ex-FM added: "Like most posh boys, given half a chance, he'll run away from a fight. Nicola Sturgeon would eat him for breakfast in a debate."

In other exchanges, the PM rebuked the Labour leadership for wanting to do a deal with the SNP, which "wanted to strip the country of its defences"; a reference to the Nationalists' desire to scrap the Trident nuclear deterrent.

"That is the risk we face; no Trident, no protection for our country; defence stripped bare by a Labour Party in hock to the SNP."

Elsewhere, Penny Mordaunt, a junior Tory Minister, also noted: "The consequence of a Labour-SNP alliance in Downing Street would be the end to our nuclear deterrent."

Meantime, after Tory peer Lord Grade, the former head of the BBC and ITV, weighed into the leaders' debates row, accusing channel bosses of breaching impartiality rules by threatening to stage the debates without Mr Cameron, Lord Hall, the BBC's Director General insisted: "We are not bullies."

He stressed the televised clashes would be an "important addition to democracy," adding: "Let us see if with a little bit of goodwill we can make something happen."