By Andrew Osborn
LONDON (Reuters) - A post-election deal between the opposition Labour Party and Scottish nationalists to rule Britain could break up the country, former prime minister John Major warned on Friday, as nationalists stepped up their efforts to woo Labour.
Major's comments underline how Britain's most unpredictable election since the 1970s is stoking uncertainty about who will govern the world's sixth-largest economy after May 7. Polls show neither of the main parties is likely to win outright.
Labour face heavy losses in Scotland at the hands of the Scottish National Party (SNP). A deal between them could be Labour's best chance of entering government.
Major, a member of Prime Minister David Cameron's Conservatives, has left frontline politics and styles himself as non-partisan on issues of national interest. He said Labour had a duty to all Britons to rule out such a deal.
"If such an alliance were to happen, the consequences could be profound. Sworn enemies would have come together, with separate motives and agendas, and the aftershock would be felt by us all," he wrote in the Daily Telegraph newspaper.
"The SNP would enter into any agreement with Labour with one overriding aim: to break up the United Kingdom. Slowly, but surely, they would try to prise the UK apart. And that is why Labour must reject it."
Major's warning came as the SNP is stepping up its efforts to woo Labour, by dropping a demand that it abandon plans to renew Britain's submarine-based nuclear deterrent in exchange for a deal.
Nicola Sturgeon, the SNP leader, told the Guardian newspaper her own party would oppose renewal of the deterrent, but shelved an earlier demand that Labour do the same.
The SNP lost last year's independence referendum, but its support has soared. It has indicated it would, in exchange for concessions, prop up a minority Labour government.
Labour, which holds its Scottish conference this weekend, insists it will win outright and is not seeking such a pact. But it has failed to rule one out, despite growing pressure from some of its own lawmakers.
It has appointed a new leader to its Scottish chapter to try to head off what could be a crushing defeat in Scotland. But polls show its decline is so serious that it might even lose its safest seat in Scotland, that of former prime minister Gordon Brown, who is retiring .