Alex Salmond is facing growing pressure to set out alternative plans for a Scottish currency from senior nationalists and allies after the three main Westminster parties snubbed a deal on sterling.

Leading nationalists such as Jim Sillars and the Scottish Green leader, Patrick Harvie, said the first minister had to openly pursue a "plan B" for an independent Scotland after the chancellor, George Osborne, led a co-ordinated attack on Salmond's sterling zone plan on Thursday.

Their demands followed calls from prominent, neutral business leaders from the Scottish Chambers of Commerce and Institute of Directors for clarity on Salmond's alternative currency plans, increasing the Scottish government's isolation on the issue.

Harvie, the most senior non-Scottish National party politician on the board of the official Yes Scotland pro-independence campaign, said he favoured a new, separate currency pegged to sterling, either on independence in 2016 or soon afterwards.

He said: "We should be looking at the option of an independent currency and deciding whether that's possible or achievable in a timescale by 2016."

Harvie agreed with Salmond that Osborne's rejection was a campaigning ploy likely to change after a yes vote, but said there was a clear risk that a sterling zone would fail to materialise, and that the stakes were now higher after Osborne's intervention.

He said: "A currency deal might be possible and might be even a good short-term position, but it's not a guarantee, so the Scottish government needs to be clear what the other options are."

Sillars, a former deputy leader of the Scottish National party, said a substantial number of SNP activists and some MSPs agreed with him that the new state should immediately introduce the Scottish pound.

He said the Scottish pound, made viable by the country's strong exports and North Sea oil, would be pegged to sterling for five years before floating.

Sillars, a former ally of the first minister who is re-emerging as a senior voice among traditionalist nationalists after publishing a new leftwing manifesto for independence, In Place of Fear 2, was more critical of Salmond's insistence on a sterling zone.

He said: "Alex has almost put himself in the position of damaging the yes side for which he has fought all his adult life."

Sillars said Osborne was justified in rejecting a sterling zone. "It takes two countries to create a currency union. If one country of 55 million people says we don't want it, there's no way 5 million people can enforce it. It's a sovereign decision by England, Wales and Northern Ireland. That's their decision and they're definitely entitled to do that."

Referring to Salmond's claim that Osborne was guilty of "bluff, bluster and posturing", Sillars said: "The message is quite clear, it [a currency union] is not going to happen, so saying 'bullying and bluffing' isn't a policy response; a policy response is having a plan B."

Harvie's stance was echoed by Ian Blackford, a former SNP party treasurer and senior investment banker, who also backed a Scottish pound closely pegged to sterling and run by a currency board, with strict controls on public spending.

Blackford said Salmond was right to pursue a currency union, but he needed to work on an alternative plan. He said: "In the unlikely event that the currency zone can't be delivered, it's perfectly sensible to then look at what can be achieved as a fall-back position. Adopting a currency board with a fixed position to sterling would be eminently sensible."

Speaking on BBC Radio Scotland on Friday morning, Salmond resisted heavy pressure to spell out his alternative currency plans by repeating that he would continue campaigning to force the UK parties to accept a sterling zone with the UK.

But he softened his stance by agreeing there were other options post-independence, set out by his government's fiscal commission of economic advisers, including sterlingisation (using the pound informally), setting up a new currency and joining the euro.

Pressed to confirm he had no alternatives, Salmond said: "No, I didn't say that." He later added: "The fiscal commission working group set out a range of options for monetary policy for an independent Scotland. They're there and have been there for the last year. They recommended that the best one was a sterling area and that's the one we're articulating and we will continue to articulate: the plan A."