Ed Miliband has ruled out any coalition deal with SNP that would see Scottish ministers in Westminster.
The Labour leader insisted the party would not form a Government using MPs from north of the border if Labour did not win a majority in May.
He said: “Labour will not go into coalition government with the SNP. There will be no SNP ministers in any government I lead.”
Crucially, however, he stopped short of ruling out a looser arrangement that would see the SNP offering limited support to shore up Labour.
It would leave the door open to the two parties entering into a “confidence and supply” arrangement, which would see the SNP vote through Budget proposals and vote for the Government in a no confidence vote.
Scottish First Minister Nicola Sturgeon said she would be willing to “strike deals with a minority Labour UK Government” and added she “cannot see for the life of me” why Mr Miliband would want to rule out a coalition with the SNP.
She said: ” … working with Labour, in a looser arrangement, I certainly wouldn’t rule out because I want to see SNP MPs being in the House of Commons arguing for and pushing for progressive change.
“I cannot see for the life of me why Labour wouldn’t want to contemplate the possibility of working with the SNP to keep the Tories out of office.”
There has been increasing pressure on Labour to rule out such a coalition from the Conservatives, who have made it clear in attack posters they are intend to campaign on the prospect of a power-sharing agreement between the two parties.
The latest poster saw a picture of Mr Miliband in the jacket pocket of the former Scottish First Minister Alex Salmond, who is running for MP in May.
It has also caused concern for Labour MPs campaigning in Scotland where the party is expecting to suffer heavy losses to the SNP.
Writing in the Scottish newspaper the Daily Record on Monday morning, Labour’s chief election strategist Douglas Alexander also said the party did not want a coalition with the SNP.
“The SNP have already said they don’t want a coalition with Labour. As Labour, we don’t want a coalition with the SNP. The coalition we want is with working people across Britain to change who our economy works for,” he said.
Mr Alexander listed a number of policies over which the SNP and Labour were at odds.
He said the nationalist party did not back Labour’s plans for a mansion tax to pay for extra NHS staff or proposals to fund measures to help children going from school to work with a cut for pension tax relief for high earners.
On Sunday, the shadow chancellor Ed Balls said: “We don’t want any deal with the SNP; it’s not part of our plans. It’s nonsense.”
Labour has looked to pressure the Tories over a deal with UKIP that could see the party hold on to power.
At the weekend Nigel Farage offered a confidence and supply arrangement with David Cameron’s party in return for an immediate referendum on Britain’s continued membership of the European Union – Mr Cameron has said there will not be a vote until 2017.
However, speaking on the Andrew Marr show George Osborne said the idea was “total nonsense”.