Nicola Sturgeon has challenged David Cameron to start talks on an "orderly and co-operative" handover of power to an independent Scotland, despite support for independence trailing in opinion polls.

Sturgeon, the deputy first minister of Scotland, has told the UK government she believes there could be a comfortable handover within 15 months of a "yes" vote in the referendum next year. That would allow an independent Scotland to be formally declared in March 2016.

Her letter to Michael Moore, the Scottish secretary in the UK government, follows an electoral commission recommendation last week that both governments start early on work setting out the steps which would follow both a "yes" and "no" vote in the referendum.

However, her attempt to seize the initiative – designed in part to portray her opponents as obstructive and defensive – was ridiculed by Alistair Darling, the former Labour chancellor and chairman of the pro-UK Better Together campaign.

He told BBC Scotland that the proposal lacked any credibility and accused Sturgeon of being deliberately misleading about the complexities of splitting Scotland from the rest of the UK.

He added that she had no mandate to assume Scotland would back independence: recent opinion polls had established only a minority of Scots would vote yes at the referendum. The polls put support for independence at between a quarter and one-third of voters.

"What they are saying is that in less than a year you can break all the ties of the past and you can fix something entirely new," Darling said.

"When you consider the currency, how you divide pensions, how we allocate debt, defence, let alone Europe, these are all issues in which the Scottish government is not going to tell us their position until the end of this year, less than 10 months before a referendum."

Announcing publication of a Scottish government paper on this transition, Sturgeon said there was ample precedent for a swift transfer of power: of the 30 or so countries which had become independent since 1945, that process took an average of 15 months to conclude.

She said the co-operative and mature discussions between both governments which led to the Edinburgh agreement – which transferred legal authority to Holyrood to hold the referendum – was a model for those talks.

Under her model, all Scotland's parties – including those hostile to independence – and civic institutions would collaborate in those talks with the rest of the UK. The laws enacting independence would be written and approved by the Scottish parliament, while Westminster's role would be solely to dissolve the act of union of 1707.

"Today's paper provides the people of Scotland with a clear roadmap as to how Scotland would make the journey from a devolved system of government with the levers of power retained at Westminster, to a nation in which the powers of our national Parliament are complete and in which the people are sovereign," Sturgeon said.

"There is no reason that talks on the process required to make Scotland an independent country – if the people of Scotland make that choice – cannot begin now and be conducted in the same constructive and co-operative manner that would lead to a smooth transition."

Sturgeon added that this timetable, which would legally confirm that the May 2016 Scottish parliamentary elections would be to run an independent parliament and state, included a commitment to introduce a legally-binding written constitution for Scotland.

Moore described the move as another example of the Scottish government's "endless distractions" from actually setting out the specific structures, policies and details of independence. He said the UK government would soon set out its views on the mechanics of implementing a "no" or "yes" vote.

"Once again, they are devoting their energy to the picture frame when they don't have a painting to put in it," Moore said. "We haven't even got a date for the referendum, let alone any detail on what independence would mean for people in areas like the economy, welfare, energy and financial services."

The proposal on a constitution was welcomed by Patrick Harvie, co-leader of the Scottish Green party, the Scottish National party's senior partner in the pro-independence movement, and Allan Miller, chairman of the Scottish Human Rights Commission.

Harvie said the timetable proposal would help political leaders and voters debate the implications of the referendum. But he added: "It remains unclear why the government is proposing the transfer of sovereignty two months before the election of an independent parliament, and I'm still concerned to ensure that the parliament has the capacity to hold an independent government to account from day one."

Willie Rennie, leader of the Scottish Liberal Democrats, said: "The SNP have hopelessly underestimated the scale and complexity of this. They would have to negotiate over 14,000 international treaties, a currency, the division of assets, membership of Nato and the host of international organisations."

"To say they will bang all this through in just 16 months is absurd. This will give most people in Scotland the shivers and fuel suspicion that the SNP are just making it up as they go along."

Scottish Labour and the Tories said Sturgeon's short timeframe [for independence] had been fatally undermined by further evidence that the EU would not begin talks on Scotland's EU membership until after independence was declared.

They said a new formal answer to both parties from José Manuel Barroso, president of the European commission, showed that the EU was unable to start talks with a new member until that state actually existed.

While confiming he could only give a precise answer on Scotland's status if the UK government asked for one, Barroso said: "Scenarios such as the separation of one part of a member state or the creation of a new state would not be neutral as regards the EU treaties. [A] new independent state would, by the fact of its independence, become a third country with respect to the EU and the Treaties would no longer apply on its territory."

Ruth Davidson, the Scottish Tory leader, said: "It makes a mockery of today's document, so much of which is based on the assumption that a separate Scotland could negotiate its terms of entry to the EU during the transition period of breaking away from the UK.

"It is now crystal clear that Scotland would have to renegotiate its way back into the EU only after its application was accepted."

Alyn Smith, the Scottish National party MEP, said Scotland's situation was different, as it was already de facto within the EU as a constituent part of the UK and was already entirely compliant with EU laws covered by the "aquis", the tests and duties required to be met by any new member

"We'll commence negotiations concurrently with the EU and UK, and both dates will take effect simultaneously. We are already compliant with the aquis, we are already part of the EU territory, so the scenario the Tories' paint is not the scenario we'll be in, and their suggestion of a queue is absurd," he said.