Alex Salmond has revealed he plans to stage his independence referendum in the autumn of 2014 after he resisted UK government demands to set an early date for the historic poll.

The first minister's disclosure – which came as the subject was being debated in the Commons – followed David Cameron's warning that Salmond must get the UK government's approval if he wanted to hold a lawful referendum or face years of legal conflict in the courts.

Salmond indicated, in a notably conciliatory tone, that he could accept Cameron's offer of a deal to legally authorise the referendum on Scottish independence, but only if that offer was unconditional.

Speaking on Sky News, the first minister said autumn 2014 "was the date that allows everything to be put in a proper manner on the most important decision in Scotland for 300 years. That date will allow the Scottish people to hear all the arguments.

"This has to be a referendum which is built in Scotland, which is made in Scotland and goes through the Scottish parliament. If the Westminster government sticks to that, we won't have too many fights about it."

Salmond's chief spokesman said they were "totally confident" that the Scottish parliament already had the powers to stage an advisory or consultative referendum, but said it would accept Cameron's offer of explicit legal powers to hold the poll only if there "no strings attached".

"The terms of the referendum will not be dictated by the UK government: those days are over," the spokesman added.

In a further development, it emerged that Salmond's hopes of staging a multiple option independence referendum are likely to be significantly boosted by a new non-party initiative to campaign for greater powers for the Scottish parliament.

The Guardian can reveal that a group of civic leaders, including senior figures in the Scottish voluntary sector, trade union movement, churches and business, are in the final stages of launching a new movement to campaign for a so-called "devolution plus" or "devo max" option for Holyrood.

Their initiative, due to be formally unveiled later this month, could greatly increase the chances of Salmond's referendum including a second question on extending Holyrood's powers short of full independence if he presses on with the poll. He plans to publish a final bill setting up the referendum in January 2013.

But in a significant blow to those ambitions, Salmond was warned by the UK government he faces huge legal obstacles if he tries to stage any independence referendum without the agreement of the UK government.

Signalling they would only support a single-question poll, David Cameron, the prime minister, and Nick Clegg, his deputy, said in a statement on Holyrood's legal powers that they had "grave concerns that the Scottish government's proposals for a referendum would not be lawful".

Their intervention intensified the row over Scotland's constitutional future as two of the most senior figures expected to join the campaign for "devolution plus" publicly rejected Cameron's demand that the referendum should include only one "yes" or "no" question on independence.

Martin Sime, chief executive of the Scottish Council for Voluntary Organisations, the umbrella organisation for 1,300 charities, said Labour, Lib Dem and Tory leaders were wrong to demand a single question poll: a majority of Scots favoured giving Holyrood greater autonomy within the UK.

"All this soundbite stuff about 'yes' or 'no' is being done for marginal short-term political gain, including the demand we should sort it out now," Sime said.

"Why should we sort it out now? We should do this properly. What matters is that we have a proper debate about the future governance of Scotland, and if politicians can't do that, then civil society will."

Grahame Smith, the general secretary of the Scottish Trades Union Congress, which has historically had very close links to the Labour party, said he wanted the body set up to run a referendum to consider "whether a credible third option has emerged and, whether, and in what way that additional option might be put to the Scottish people".

Michael Moore, the Scottish secretary, confirmed that the UK government's legal advice was that the Scottish parliament currently had no legal authority to stage the referendum in any form, despite Salmond's insistence that it would be simply indicative.

He said the UK government's law officers, including the Attorney General Dominic Grieve, had ruled that under the Scotland Act 1998, Holyrood was explicitly barred from passing any measure which affected the UK's constitution. If Salmond tried to stage a referendum the UK government or private citizens could get it struck down as unlawful by the UK supreme court.

And, in a deliberate move to prevent Salmond staging a multi-option referendum, Moore said the UK government would temporarily give Holyrood the legal authority to stage that referendum but only if it was a single "yes" or "no" question about independence, and only if the referendum was run by the UK Electoral Commission.

Buried in the UK government's consultation paper was the disclosure that it was also legally able to stage the referendum itself, if Salmond refused the offer from London. This measure would cause uproar amongst Scottish nationalists and the centre-left in Scotland.

Moore veered away from this politically-explosive option in his Commons statement. He insisted he wanted to enable a "legal, fair and decisive" referendum to take place.

"As a Scot, I think it's vital that the Scottish people make a clear decision about our future in the UK; it should be a decision made in Scotland by the people of Scotland," he said. "But at present there is a lack of clarity about the referendum, its outcome and what the implications of that outcome would be."

The UK government's offer was broadly welcomed by the Scottish Labour party, putting further pressure on Salmond to accept demands for a single question referendum.

Johann Lamont, the Scottish Labour leader, attempted to further isolate the first minister by calling on him to set up cross-party talks with his opponents and civic leaders to agree a mutually-satisfactory referendum date.

She accused Salmond of revealing the 2014 date in a panic. "The first minister should hold cross party talks – including all quarters of civic Scotland – to discuss these details including the date," she said.

"It is also rather sad that a first minister of Scotland, keeps his plans for the constitutional future of our nation secret, doesn't tell the Scottish parliament, but goes on the TV because a debate at Westminster might keep him out the headlines.

"We deserve better. This is about the future of Scotland, not the profile of Alex Salmond. This issue deserves calm consideration – not a panicked response from a panicked first minister."