Scotland's new Labour leader Jim Murphy said today was a "fresh start" for the ailing party as he declared next year's general election would be a referendum on whether David Cameron remains as Prime Minister.

Mr Murphy said he was confident under his leadership Labour would not lose any Scottish MPs to the the SNP in May's election.

The number of seats Labour wins north of the border will be crucial to Ed Miliband's chances of becoming the next prime minister.

But a new opinion poll today in Scotland put his party 20 points behind Nicola Sturgeon's SNP in Westminster voting intentions, with 47% support ahead of 20% for Labour.

The East Renfrewshire MP said his election as leader, together with Kezia Dugdale's victory in the contest for the deputy leadership, gave the party a new start.

The party's new Scottish leadership team was announced at the Glasgow Emirates Arena, with Mr Murphy declaring: "'This is a fresh start for Scottish Labour.

''Scotland is changing and so too must Scottish Labour. I'm ambitious for our party because I'm ambitious for our country.''

The leadership contest had been sparked after Johann Lamont stood down, accusing party colleagues in Westminster of treating Scotland like a "branch office"

Mr Murphy, a former Scottish secretary, picked up a total of 55.7% of the vote in the election, putting him ahead of Holyrood health spokesman Neil Findlay, who secured 34.99% and former Scottish Executive minister Sarah Boyack, who came in third with 9.24%.

Afterwards he said he would "take the energy of this contest and use it to unite the Scottish Labour Party".

Despite the surge in support for the SNP since September's independence referendum, Mr Murphy insisted he was not daunted by the challenge that is facing him.

With the general election taking place in less than five months, he said: "What's at stake is the governance of the United Kingdom.

"Who forms the next government of the United Kingdom will partly be determined by the Labour Party in Scotland picking itself up, having a sense of confidence and winning again. And that's what I'm going to do.

"In the UK election next year the biggest party gets to form the government, any seat that Labour loses in Scotland reduces the chances of the Labour Party being the biggest party and gives a chance to David Cameron to lead another government.

"This isn't about the future of the union, it's about the governance of the United Kingdom."

He added: "Any seat that the SNP win from Labour - and I'm confident that we will hold all that we have - any seat the SNP win from Labour reduces the chance of Labour being the biggest party and increases the chances of David Cameron having the biggest party. I'm determined that that won't happen.

"We've had one referendum in September, we're going to have another referendum in May and it's a referendum on whether Scotland genuinely wants rid of David Cameron.

"So a second referendum in a year - this time it's not about Scotland's constitutional future, it's about David Cameron's future."

Mr Miliband congratulated both Mr Murphy and Ms Dugdale on their appointments and said: "I look forward to working with both Jim and Kezia to secure a Labour government in Westminster next year and Holyrood the year after."

The UK Labour leader added: "Jim showed in the referendum campaign that he is a fighter. He showed in the leadership campaign that he is a leader. I am going to be standing shoulder to shoulder with Jim in the campaign to get David Cameron out at the general election."

While Mr Murphy's career so far has been at Westminster, his appointment as Scottish Labour leader means he is now looking for a seat at Holyrood, in the hope of becoming first minister after the 2016 Scottish Parliament elections.

"My commitment will be to be in the Scottish parliament by 2016, to be Labour candidate for first minister in 2016," he said.

"If we can do it sooner than that I will."

As to whether he will stand for Westminster in the upcoming general election, he said he would "make that a lot clearer in the New Year".

Despite the challenge the party and his leadership will face in that election, Mr Murphy said: "I'm not daunted at all, I'm determined. I said at the beginning of this contest it is not a kamikaze mission, I know that we can win, I'm confident we will."

He urged those who had voted for independence in the referendum to back his party in May, claiming he had more in common with ''the values of the many hundreds of thousands who voted Yes in the referendum than with many of the political leaders who campaigned for No''.

He told Yes supporters: ''We share a commitment to a fairer Scotland, we share a commitment to social justice, we share a sense of values. We disagreed on one vote on one day but we shouldn't allow that to divide our nation.

''We should allow Yes and No to unite us - yes to a fairer Scotland, yes to a Scotland of limitless potential and no to a Scotland of generation inequality.

''That is our challenge, that is our mission.''

He accepted there was a "huge amount of work to do in a really short period of time" ahead of the general election.

Mr Murphy pledged: "I'm going to build a coalition of voters, those who voted Yes, those who are aspirational, those who want to build a more prosperous economy, who want to generate and earn wealth, and build a coalition of those voters."

Although Labour is trailing the Scottish nationalists in the opinion polls, Mr Murphy said these would turn around.

He added: "Someone said to me the other day that polls are just predictions but they're also there to be proven wrong. That's what I'm determined to do."

Ms Sturgeon, the Scottish First Minister, congratulated both Mr Murphy and Ms Dugdale on their new roles.

"I know that the challenges of leadership are never easy, so I offer my best wishes for the job they have ahead," the SNP leader said.

"While we will undoubtedly cross swords often in the months ahead, my door is always open to those who wish to find common ground and work together in the best interests of people in Scotland - something I hope we will have the opportunity to do."