Cameron and Salmond sign the daddy of all British referendums

Britain is about to experience one of the longest and most important referendums in its history, after David Cameron and Alex Salmond signed off on a two-year campaign over Scottish independence.

The two leaders met in Edinburgh today, where they agreed on a one question, in-or-out referendum in which 16-and-17-year-olds are allowed to vote in autumn 2014.

The Scottish parliament will have the power to formulate the question asked, although it will need to be signed off by the Electoral Commission, which can change the wording if it is deemed biased or unclear. They will also set funding limits.

"I don’t want to rip anything up," Salmond told reporters.

"We're not in the business of ripping things up. We're in the business of building a new relationship between the people of these islands."

"This marks the beginning of an important chapter in Scotland's story and allows the real debate to begin," Mr Cameron said.

"It paves the way so that the biggest question of all can be settled: a separate Scotland or a United Kingdom? I will be making a very positive argument for our United Kingdom."

Many commentators think Westminster has outplayed the Scottish National party (SNP) leader. Even though London gave ground on the length of the campaign, the extension of the franchise and the formulation of the question, it is thought negotiators were only really interested in limiting the referendum to one question.

A third 'devo-max' option, which would have handed the Scottish executive more powers, would almost certainly have won.

Current polling puts support for independence on 28% and those backing continued union on 53%.

"All YouGov's evidence from the past four years is that independence is a minority passion north of the border," YouGov boss Peter Kellner said.

"Even as the SNP was surging to victory last year, Scots told us by two-to-one that they wanted to remain within the UK."

The SNP is hoping that detailed plans for an independent Scotland and the extension to the franchise will boost their support, although younger voters are famously the least disciplined at turning out to vote.

The nationalists are also hoping the 2014 Commonwealth Games in Scotland will give their campaign a boost.

Most pollsters will be particularly focused on the last months of the campaign, when any game-changing event could be enough to swing the vote decisively either way.