David Cameron's comments came as he faced criticism from some Conservative donors and speculation that Boris Johnson is being lined up to take over as leader.

The London Mayor and Westminster candidate has dismissed the reports as "nonsense".

Mr Cameron also defended his warnings of a minority Labour government propped up by the SNP, arguing "it is the right and responsible thing to point out the dangers of that".

Responding to suggestions, the campaign had been "threadbare" and "negative", Mr Cameron told Murnaghan: "People talk about this campaign but there's nothing more positive than let's get another 2 million jobs, fund another 3 million apprenticeships, let's let you to keep more of your own money to spend as you choose, let's build those homes you want to live in, let's make sure there are good schools for your children.

"That's the most positive vision could possibly be."

As the party launched a fresh economic offensive, Mr Cameron said: "In our Queen's Speech it will all be about our economy, the economy is not lines on a graph, it's people's jobs people's pay.

"It's at the heart of everything, it's at the heart of election campaign, and we're going to focus on that to the exclusion of everything else for the next eleven days."

The Conservative poster campaign has, however, pushed further the other central message about the SNP's parliamentary influence over a possible Labour minority government.

The former First Minister of Scotland Alex Salmond is depicted as a pickpocket about to grab a wallet from an unsuspecting English voter with the worlds "don't let the SNP grab your cash".

The Chief Secretary to the Treasury Danny Alexander attacked the conservative campaign as a "total disgrace".

But Mr Cameron said: ""I am simply pointing out a fact of this election which is its become apparent, and you can ask any expert, that Labour is likely to suffer very badly at the hands of the SNP in Scotland and that has consequences for everybody in the rest pf the country.

"Because it means Ed Miliband can only form a government with the assistance of the SNP. Now that I think is a very disturbing development for our country because the SNP don't come down to Westminster to make our government stronger or make our country stronger. They come down to Westminster to break it up.

"And I think it is the right and responsible thing to point out the dangers of that.

"The point is I don't just made this as an observation, I have an answer.

"There are only 23 more seats the Conservatives need to win an overall majority to stop this from happening."

Mr Cameron also responded to his "brain fade" over his favourite football team.

He said: "I've been an Aston Villa fan all my life, I literally opened my mouth and I was going off-piste about the fact that in Britain you can be a supporter of the West Indies, a supporter of Manchester United, and a supporter of Team GB, and I was then busking about other things you can support and was.

"I don't know what happened to me, it was just one of those things... I suppose it is just the campaign, by the time you have made as many speeches as I have on this campaign all sorts of funny things start popping out of your mouth".

The PM declined to name the year that Aston Villa had won the European Cup, but recalled players such as "Andy Gray and Brian Little, and that's what got me going ... But I don't go very often, in fact I haven't been for years and years, but I try and keep up".