After crisis warnings, UK's Labour says no deals with Scottish nationalists

* UK faces election on May 7

* Labour and Conservatives level in polls

* Possible Labour-SNP tie-up one option

* Labour moves to rule that out (Adds Sturgeon, Cameron, Murdoch)

By Andrew Osborn

LONDON, April 26 (Reuters) - Britain's Labour Party on Sunday ruled out a post-election deal with Scottish nationalists after Prime Minister David Cameron's Conservatives said such a tie-up could spark the biggest constitutional crisis since the 1936 abdication of the king.

The policy shift, by Labour leader Ed Miliband, was designed to blunt what has become Cameron's main attack line before the May 7 election, a contest that is shaping up to be the country's closest and most unpredictable since the 1970s.

The stakes are unusually high: Cameron is promising a European Union membership referendum which could see the world's fifth largest economy leave the trading bloc, while Scottish nationalists are pushing for a deal with Labour which opponents fear could pave the way for another independence referendum.

With Labour and the Conservatives level in most polls and neither on track to win an outright majority, Cameron has gone from talking up his economic achievements to warning that a tie-up between Labour and the pro-independence Scottish National Party (SNP) could lead to a UK break-up.

Miliband had previously ruled out a formal coalition with the SNP, but had left the door open to a looser arrangement. On Sunday, he went further, ruling out an informal deal known as a 'confidence and supply' arrangement.

When asked if he would negotiate with the SNP after the election if he failed to win a majority, Miliband told the BBC's Andrew Marr Show: "I'm not interested in deals no."

"I want to be clear about this. No coalition, no tie-ins. I'm not doing deals with the Scottish National Party."

The shift - which rules out Labour ruling as a minority government relying on the SNP in parliament on an issue-by-issue basis - narrows Miliband's room for manoeuvre and reflects his concern that the Conservative message is "cutting through," in particular to English voters worried about Scottish influence.

It suggests that Labour would adopt a different strategy if it fell short of a majority and try to rule as a minority government challenging the SNP, which is also left-wing, to vote it down.

The calculation would be that the SNP would not do so because it would struggle to explain to its own supporters why it had brought down a left-wing government to make way for a right-wing one.

CONSTITUTIONAL CRISIS?

The Conservatives, who have struggled to turn an economic upturn into extra voter support, have stepped up their warnings about what they say would be the risks of a Labour-SNP deal.