Tony Blair has urged Labour to defend Scotland from the "constitutional nightmare" of an SNP government.
Mr Blair put forward his defence of the Union
The prime minister was addressing his last Scottish party conference in Oban, where he told activists to redouble their efforts against the Nationalists.
He said that without Labour, Scotland would never have seen the success it had over the past 10 years.
However, the SNP said Mr Blair was "a crushing liability" for his party's Holyrood election campaign.
Mr Blair led the attack on independence in his speech while First Minister Jack McConnell will use his address on Saturday to concentrate on his own policy agenda.
The prime minister warned against a "constitutional nightmare" where the UK turns in on itself.
He stressed that the choice in next May's election would be between a Labour government or an SNP one which plans to introduce an independence bill within 100 days.
Mr Blair told party members: "Already they are publishing plans for separation - separate currency, separate pensions and social security systems, leaving Nato."
He went on: "The fact they are saying it and with utter precision shows they are deadly serious and would do it."
The choice for voters was portrayed by Mr Blair as between "the basest metal of politics, the politics of grievance" and "the politics of progress".
"The reason I detest this narrow nationalism is not because it engenders fear, but because it squanders hope," he said.
Rallying the faithful for the coming election campaign, he urged delegates to go out with the message that Labour had delivered for Scotland as part of the United Kingdom.
Before Mr Blair made his speech, the SNP published details of a poll it had commissioned from YouGov.
It asked 1,034 people whether they would be more or less likely to vote for Labour if Mr Blair was still prime minister at the time of the Holyrood vote.
The SNP said 24% said they would be less likely to vote Labour, 7% would be more likely to vote Labour and 60% said it would make no difference.
SNP leader Alex Salmond said the prime minister's attempts at scaremongering over nationalism were simply "cauld kale het up" by a desperate, discredited man.
He said: "The hysteria of his attack on the SNP is a tribute to our success in pinning Labour in Scotland on the political ropes, as all the recent evidence shows."
"Blair bashing the SNP is the ultimate act of 'boomerang politics', as it will simply return to haunt him.
"His attacks reinforce our position as the only challengers to Labour next year, and undermine any pretence that Labour in Scotland are capable of standing on their own two feet without interference from London."