SCOTLAND could declare ­independence at a future date without the need for a referendum, Alex Salmond has said, as he paved the way for his ­successor to pursue a new strategy for achieving the SNP's goal.

The First Minister said there were many routes to independence after Scots voted to stay in the UK in last week's referendum.

He also accused older Scots, whose votes swung the referendum in favour of No, of impeding progress for younger generations.

His comments provoked a furious reaction from politicians who campaigned for a No vote in last week's historic poll.

Scots Labour leader Johann Lamont accused him of attempting to overthrow the will of the Scottish people just days after voters decisively rejected independence.

Mr Salmond spoke out as Downing Street gave a guarantee that David Cameron will honour his commitment to give Scotland more powers.

A senior No 10 source responded to a claim from Mr Salmond that voters had been tricked by the promise of more powers, which then appeared to be linked to more devolution for England, by making clear one was not conditional on the other.

"This will happen come what may. We are absolutely clear; Mr Cameron is not reneging on his commitment to Scotland," said the source.

However, the pledge from Mr Cameron risks a backlash from Tory right-wingers and he has invited some of his party's most vocal backbench critics to an "English votes for English laws" summit at Chequers today, seeking to head off a potential rebellion.

Mr Salmond earlier raised the possibility of independence without a referendum as the SNP began to consider its future without him at the helm.

He announced his resignation within hours of Thursday night's defeat and looks certain to be replaced by Nicola Sturgeon when he steps down at his party's annual conference in November. Speaking on Sky News yesterday, he said the new leader could abandon the policy he had pursued of seeking a mandate for independence from a referendum.

He said Scotland could declare ­independence after acquiring sufficient power at Holyrood to be independent in all but name, adding: "Many countries have proceeded through that route."

The First Minister also highlighted the parliamentary route to independence, raising the possibility of the SNP reverting to its previous policy of seeking a mandate in a General Election.

Jim Sillars, the former deputy SNP leader, tweeted: "Let Yes assert new indy rule - no more ref - majority votes and seats at Holyrood 2016 enough."

Before 2000, when Mr Salmond first promised a referendum on independence, the SNP said winning a majority at Holyrood, or of Scottish seats at Westminster, would give the party a mandate to negotiate to leave the UK.

The First Minister said yesterday: "The referendum route was one of my choosing, it was my policy, I thought that was the right way to proceed but of course there is a whole range of ways Scotland can improve its position in pursuit of Scottish independence.

"A referendum is only one of a number of routes."

His referendum policy, which was resisted by the so-called "fundamentalists" in the SNP prior to its introduction, has been key to the party's electoral success. The party has won two successive Holyrood elections after telling Scots they could back the SNP even if they did not support independence.

Nationalist MSP Marco Biagi yesterday called on the party to adopt a gradualist approach to independence by seeking further powers for Holyrood rather than offering a "direct route".

Writing in a Sunday paper he said: "A pledge of another referendum in the next SNP manifesto is now inconceivable."

Others in the party believe it would be possible to hold a second independence referendum in 2017 if Mr Cameron wins next year's UK election and holds his promised vote on the UK's membership of the EU. If the UK votes to leave the EU without majority support in Scotland, some Nationalists believe the result would justify another independence vote.

But Ms Lamont said: "Having decisively lost a democratic referendum on independence, Alex Salmond is now suggesting the Nationalists can ignore the sovereign will of the Scottish people.

"His words are fundamentally undemocratic and an insult to the people of Scotland."

She added: "Alex Salmond lost. It is not for him to try to overthrow the will of the Scottish people in some sort of coup."

* Gordon Brown will not return to a frontline role in British politics, Labour leader Ed Miliband has said. The former Prime Minister played a dramatic role in the Scottish independence referendum debate, galvanising the No campaign with a series of last-minute interventions proposing a timetable for further devolution.

Asked whether he expected Mr Brown to return to the front line, Mr Miliband told BBC1's Andrew Marr Show: "I don't think that's going to happen. He played an important role in the referendum, but he is not going to come back to frontline politics in Britain."