Senior Labour MP Alan Johnson has called on Ed Miliband to rule out a deal with the Scottish National Party to secure power after the May 7 general election.

Mr Miliband is coming under pressure to rule out a coalition, or an arrangement under which the SNP would prop up a minority Labour administration, after Conservatives launched a poster showing him in the pocket of Alex Salmond.

Former home secretary Mr Johnson, who formerly served as shadow chancellor in Mr Miliband's frontbench team, said he believed most Labour MPs in Scotland wanted him to rule out a deal.

Asked if the leader should leave open the possibility of a post-election arrangement with the SNP, Mr Johnson told BBC1's This Week: "I think he should rule it out. I can't see a downside to it.

"We can't possibly do a deal with a party that wants to be out of the United Kingdom and give up our nuclear deterrent.

"We can't possibly do a deal with a party on that basis.

"And also, if we said we would go into coalition, it means in Scotland that voting SNP means you can get a Labour government."

Asked if Labour supporters would have liked to see Mr Miliband rule out a pact at the party's recent Scottish conference, Mr Johnson said: "The majority of Scottish Labour MPs - I haven't spoken to anybody in the Labour Party who doesn't want him to do that and I think that's certainly the view in Scotland as well."