Alex Salmond has taken the entire Labour Party hostage and is using Ed Miliband as his poodle, the Prime Minister has said.

David Cameron continued the Tory attack on the Opposition's potential need for SNP support to govern, with polls predicting the party formerly led by Mr Salmond could increase its representation at Westminster from six to 40 or more.

Mr Salmond has also said the SNP would block any attempt by the Tories to form a minority administration by voting down its Queen's Speech.

Speaking in the final Prime Minister's Questions session of the Parliament, Mr Cameron was asked if he was alarmed by the potential demands on the UK Government by Mr Salmond.

The PM said: "As far as I can see, Alex Salmond has taken the entire Labour Party hostage and today we have got the ransom note.

"And the ransom note is very clear. It says higher borrowing, uncontrolled immigration, unfettered welfare, higher taxes and weaker defence.

"That's what is being demanded and the British people have only one way of saying no to this appalling hostage situation - and that is to vote Conservative on May 7."

Labour's Stephen Pound (Ealing North) later attempted a joke at Mr Cameron's expense, telling the PM: "Last week some of the rougher elements of the House chose to refer to you as chicken. I hope we have moved on.

"However, would you agree with me it's entirely fair to refer to you now as a lame duck?"

But Mr Cameron replied: "I'll tell you what is a lame duck and that is trying to get into Downing Street on the back of Alex Salmond's coat-tails.

"They now know they cannot win the election on their own so they are preparing to answer the ransom note - higher taxes, more borrowing, weaker defence, breaking up our union - that is what we've got to stop."

Addressing Labour leader Mr Miliband, the PM added: "Never mind talk of ducks, I'm looking at Alex Salmond's poodle."

Elfyn Llwyd, Plaid Cymru's leader in Westminster, also asked if his departure from Parliament would cause more anguish than the return of Mr Salmond.

To laughs, Mr Cameron replied: "I was quite looking forward to missing you both."