Cameron: a Labour wipe-out in Scotland will mean only the Tories can form a majority government

Speaking in Glasgow at the launch of the Scottish Conservative election manifesto, Mr Cameron insisted an overall Tory majority is "achievable" on May 7, despite polls pointing to a hung parliament.

He also used his address to warn of the "terrible prospect this country faces" if Labour and the Scottish nationalists work together to lock the Conservatives out of government.

He said such an agreement would be a "coalition of chaos" that would put the future of the United Kingdom in danger.

Mr Cameron told party activists: "We are only 23 seats short of an overall majority, and I believe that overall majority is achievable.

"What are the new facts of this election campaign. There are two new facts. Fact number one: The Labour Party here in Scotland is facing wipe-out at the hands of the SNP.

"Fact number two: The Liberal Democrats are facing wipe-out potentially in many parts of the country.

"What that means is Ed Miliband can only get into Downing Street on the back of support from the Scottish National Party, who will exact a terrible price in terms of even more borrowing, even more spending, even more unlimited welfare and even weaker defences.

"That is the terrible prospect this country faces.

"The conclusion from those two facts is this. There is only one way to stop that from happening. You can't stop it from happening by voting Liberal Democrat - they can't stop anything. You can't stop it by voting Ukip, that just makes a Labour MP and a Labour victory more likely.

"The only way you can stop this dreadful outcome is by voting Conservative in England, in Scotland, in Wales, in Northern Ireland. We are the only people that can stop this from happening."

The Prime Minister argued Tories are "the only ones that can form an overall majority, an overall majority to continue the plan that is working for Britain".

And he said the party is also "the only people who can stop this dreadful proposition of weak a Labour leader propped up by the SNP wrecking our country".

With support for Labour trailing well behind the SNP in Scotland, he said there is a "massive political fight" taking place north of the border.

Mr Cameron insisted, though, that the two parties are very similar, saying: "You have a weak Labour Party, who want more spending, more borrowing, more debt and more taxes. And the people who will prop them up, the SNP, who want even more spending, even more borrowing, even more debt and even more taxes.

"Together, they pose a clear threat to the future of our United Kingdom. A coalition of chaos. The SNP acting as the chain to Labour's wrecking ball, running right through our economic recovery."

He made clear the Tories would "never do any sort of deal with any party that wants to break up our United Kingdom" - although the SNP has also rejected any prospect of working with the Conservatives.

Mr Cameron said the Conservatives are the party "with a plan that is turning Scotland around".

Addressing voters, he said: "When you're standing in the polling booth a few weeks from now, ask yourself this: 'Why would we tear up this plan that has got us this far?'

"The answer is we shouldn't. We should see this through."

Ballot papers for postal voters will start arriving in people's homes from this weekend, Mr Cameron said.

"If you think I would do a better job as prime minister than Ed Miliband you need to vote for that outcome and not put our economy and our recovery at risk," he told the public.

"There are only two people who can walk through the door of Number 10 Downing Street. I can walk back through that door and I can carry on the work of getting this country back to work, cutting people's taxes, making it easier to own your home, getting good school places for your children, making sure Britain continues to be a success.

"Or you can have Ed Miliband walking through that door, backed by this coalition of chaos including the SNP here in Scotland.

"The way to avoid that is to vote Conservative. People must vote against the outcome they don't want - the chaos - and in favour of the outcome they do want, which is to continue with the plan which is working for our country."

He added that Labour is "facing a meltdown" in Scotland at the election, saying: "I'm responsible for many things, believe me, the total failure of Labour in Scotland you cannot lay at my doorstep."

Mr Cameron predicted Mr Miliband's party will have a "dreadful time in Scotland" and will be "replaced in many, many constituencies by the SNP".

He said: "Ed Miliband cannot become prime minister without SNP support. That has a consequence for Scotland and it has a consequence for the rest of the United Kingdom.

"It is the appalling prospect of having a Labour prime minister propped up by a group of people who don't want to be in that parliament, don't want to be in that country, and would like to see the whole thing break up - and in the meantime are going to push for an economy-wrecking borrowing, taxing and spending agenda.

"I say this because I think the country really needs to hear this, voters need to think about this, their ballot papers are about to arrive on the doorstep.

"You can stop this from happening, but you can only stop it by electing a Conservative majority government."

Scottish Conservative leader Ruth Davidson said Mr Miliband, his shadow chancellor Ed Balls and Labour's Scottish leader Jim Murphy have turned down "chance after chance after chance" to rule out working with the SNP.

She added that Scottish nationalists would have a "price list and a check list and a shopping list" for supporting Labour in government.

"We know what it is, we know that is reducing our defences, increasing the deficit, increasing borrowing," Ms Davidson said.

"But don't think that snuck in at the end of that shopping list, there won't be more things to bring us closer to the independence we all just voted against.

"That's why we need to point out to the voters of this country that that dodgy deal is being done by the Labour Party and shame on them for doing it."