Ed Miliband should shun a coalition pact with the Scottish National Party in order to keep the UK together, Sir John Major has urged.
The former Conservative prime minister has urged the Labour leader to rule out a coalition with Nicola Sturgeon, adding it was "shameful" Miliband had not already done so.
Major believes a Labour-SNP deal could lead to a break up of the United Kingdom, which he claimed to be the SNP's "overriding aim".
Writing in the Telegraph, Major said: "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?"
Major, who succeeded Margaret Thatcher as leader of the Conservative Party in 1990 and was Prime Minister until Labour's landslide 1997 election victory, said a power-sharing deal with the SNP would benefit Scots and not the whole country.
His intervention came after a Lord Ashcroft poll showed the SNP making considerable gains in Scotland and threatening some of Labour's safest seats.
Polls in eight Scottish constituencies predicted that as many as six could go to the SNP - including former Labour prime minister Gordon Brown's Kirkcaldy and Cowdenbeath constituency, where he opened up a majority of 23,000 at the 2010 election.
The study also revealed Charles Kennedy, a Liberal Democrat MP since 1983, faces a battle to keep hold of his Ross, Skye and Lochaber seat.
It also revealed that Scottish Labour leader Jim Murphy holds just a one-point lead in East Renfrewshire over the SNP.
The rise of the SNP in the wake of last year's independence referendum has also cast a shadow over the ambitions of the Lib Dems north of the border, with as many as ten predicted to lose their Westminster seats.