Conservative leader David Cameron said on Friday the party faces a "battle" in the Glasgow North East by-election.

Mr Cameron spoke during a visit with Ruth Davidson, the Scottish Conservative's candidate for the by-election, at McGhee's bakery in Glasgow.

Mr Cameron said: "Every by-election is a battle but in Ruth Davidson we have a first class candidate. I think Labour have failed people in Glasgow. I think it's time for a change. I'm the first party leader up here on the campaign trail. It's time for change and people should vote for it."

Mr Cameron toured the McGhee's factory site along with Scottish Conservative leader Annabel Goldie and David Mundell, Conservative MP for Dumfriesshire, Clydesdale and Tweeddale.

Asked what he thought about a poster made by Scottish Labour with a photo of Mr Cameron and the words "Vote SNP get Tory", he said: "I think when people start putting your opponent's face on posters it probably means... I don't know what they are doing," he laughed.

He said: "My reaction is that Ruth is going to fight a positive campaign. She's saying let's clean up politics, let's clean up expenses. She's out on the doorstep calling for support.

"We'll be fighting a positive campaign," he added.

Asked about calls for a Scottish referendum on independence, Mr Cameron said: "All the opinion polls that I see suggest that actually people in Scotland don't want to break with the United Kingdom.

"Actually when you look at what Alex Salmond has been doing, he knows that. That's why he's always talking about the process and moving a bit closer. Actually that's rubbish. I think at the end of the day when you ask people would you like to break with the United Kingdom the answer is no they wouldn't.

"What you have in me is someone who, if we win the election, will always put the United Kingdom first, will do nothing to put the union at risk and would govern showing great respect to people in Scotland.

"I've always said this when I've come to Scotland - Alex Salmond thinks somehow the election of a Conservative government is going to help him achieve his dream, he's got another thing coming," he added.

Earlier, Jim Murphy - Secretary of State for Scotland - attacked the Conservatives claiming their policies would threaten jobs.

He said: "Glasgow has never forgotten or forgiven the Tories. They are the party that brought mass unemployment to Glasgow in the 80s and 90s.

"Leading economist, Professor David Blanchflower, has warned that five million people would be jobless under the Tories. That would be 400,000 in Scotland and nearly 50,000 in Glasgow.

"David Cameron is even less popular in Scotland now than Mrs Thatcher was in the 1980s - but he is no less a threat to Scotland's families and our economy."

Earlier,  the Government formally launched the Glasgow North East by-election to fill the seat vacated in June by ex-Speaker Michael Martin - now Lord Martin.

The writ was moved in the Commons for the contest on November 12 - after a wait of nearly five months. The SNP had failed in a bid to force the poll in July before the Commons' break.