David Cameron was in Edinburgh addressing a gathering of party members.
He claimed Labour had wrapped itself in the Union flag during the referendum campaign - but was now prepared to work with the SNP, who he said would "rip up that flag given half a chance".
Voters in the UK will go to the polls on 7 May to choose their MPs.
Polls are indicating that Labour would be the party with the largest number of MPs, but unable to form a government on its own.
They also suggest that Scottish Labour MPs would lose a significant number of seats to the SNP.
In the light of those polls, Mr Cameron speculated that a vote for "anyone other than the Conservatives" risked Labour leader Ed Miliband becoming prime minister, "leading an unstable minority government".
He continued: "A vote for the SNP is a vote for Labour in government.
"Nicola Sturgeon has made clear she is up for a coalition with Ed Miliband."
Mr Cameron added that votes to the SNP and Labour would allow former nationalist leader Alex Salmond come in "through the back door".
He told the one-day conference: "Like a horror movie - he's back. Only this time he's not running Scotland, he would have the decisive say in running a country he wants to see abolished - our United Kingdom.
"Isn't it appalling that Labour won't rule out this outcome - that they would wrap themselves in the flag one minute, and the next be prepared to work with a bunch of people who would rip up that flag given half a chance?
"Spineless. Weak. Unprincipled. Short-termist. That, my friends, is all anyone needs to know about today's Labour Party."
Earlier, Mr Cameron visited BAE Systems on the Clyde where he spoke about new investments to develop the Type 26 warship.
He said 600 jobs would be safeguarded in Scotland, along with "many more across the UK".
Mr Cameron told workers:  "Investing in these warships will ensure we continue to keep our country safe, at home and abroad.
"As part of our long-term economic plan, we're not just building the most advanced modern warships in the world - we are building the careers of many young people with apprenticeships that will set them up for life."
His conference speech also touched on what the Conservatives planned to do in the future.
Mr Cameron said next on the agenda was "getting Britain back to work".
He explained: "Above all - work must always pay. That's what the benefit cap is about. That's what Universal Credit is about. And it's working.
"Across our country, there are 900,000 fewer people on the main out of work benefits.
"Here in Scotland - unemployment is down - lower than the UK as a whole."
The Tory leader finished his speech by saying "we have the brilliant team, we have the record, we have the long-term economic plan to secure Scotland's future".
He added: "We've won for Britain before - now let's win for Britain again. "
The prime minister has attacked Labour as "weak and spineless" in a speech to the Scottish Tory conference.