Former Prime Minister Sir John Major has urged Ed Miliband to rule out governing with the SNP in order to protect the United Kingdom. The ex-Tory leader said the nationalists would enter any deal with the "overriding aim" of "prising apart" the union.
Writing in the Daily Telegraph, Sir John said he was speaking as "an Englishman with a profound admiration and respect for Scotland". He argued it was "shameful" that Labour have not already ruled out a power-sharing deal.
"They would be relying on support from a party that will use every strategy it can to break free of the UK," he added.
"Labour now have to make a choice. They must summon the courage of their convictions and declare their intent. The British people - north and south of the border - do not deserve to be misled. Labour must remove any doubt. If the outcome of the general election is inconclusive, will they refuse to govern with the support of a party whose principal aim is to break up the United Kingdom?"
An SNP-Labour coalition would "penalise UK citizens outside Scotland", and worsen relations between England and Scotland, Sir John warned.
He said: "Separatism thrives on grievance, real or imagined, and the SNP would promote grievance wherever and whenever they could. They have done so for many years, and would not stop... Neither history, nor electors, would forget if Labour facilitated the break-up of the UK. We have a tragedy in the making and, by that, I do not only mean this possible alliance of political enemies. I mean something far more basic: the alienation of the Scots from the English."
On Thursday, a poll carried out by Lord Ashcroft predicted the SNP could make huge gains in Scotland at Labour's expense, including Kirkcaldy and Cowdenbeath, where former Prime Minister Gordon Brown is stepping down.
The nationalist party could also remove former Liberal Democrat leader Charles Kennedy, according to the poll, as well as Edinburgh South West, where former Labour chancellor Alistair Darling, the man charged with keeping Britain a union ahead of last year's Scottish independence referendum, is retiring.