SNP plans for a new Scottish currency have been branded a “fantasy” after Derek Mackay admitted they relied on an economic boom after independence.

The Finance Secretary said higher growth would cut Scotland’s £13bn deficit and create the right conditions for a rapid switch from sterling.

The SNP’s Growth Commission last year said it would be probably be at least a decade after independence before a new currency was viable.

It said Scotland would need to halve a 6 per cent deficit to establish market credibility, and public spending should be kept under very tight control.

Critics say the prescription would essentially continue Tory austerity.

But Mr Mackay, who is under pressure from SNP activists not to keep sterling, said a faster timetable was possible without austerity through stronger growth.

READ MORE: SNP would propose a new currency for an independent Scotland

Official forecast put Scottish GDP growth at around 1 per cent until 2023.

The Growth Commission, which Mr Mackay sat on, suggested it could take 10 years to reach 2.5% and 25 years to reach 3.5%.

But Mr Mackay told BBC Sunday Politics Scotland that after a “safe and secure transition” with sterling, independence could deliver a swift GDP boost.

He said: “We’ve got the ambition and the commitment to have an independent currency and a process to take us there. We want to grow our economy.

“Yes, we want to bring that estimated deficit down... Growing our economy is an alternative to austerity.

“We could reduce that notional deficit… accelerate the economic growth and move to that independent currency when we’re able to do so, by setting out the tests that would guide us there.

"With the powers of independence, we can stimulate the economic growth, grow our economy, get that national deficit down.”

Mr Mackay and SNP depute leader Keith Brown will ask SNP members next month to vote on whether it should be party policy to establish an independent currency, and whether an SNP government should aim to complete preparations to enable Holyrood “to take a decision on establishing a new currency by the end of the first term of an independent parliament”.

However their conference motion also says "an economic stimulus at the point of independence should be considered", meaning extra public spending or tax cuts, both of which could add to the deficit and so delay any new currency.

READ MORE: Tom Gordon: SNP’s currency plan may please members, but not voters

Tory MSP Murdo Fraser said: “This is fantasy economics from Derek Mackay and the SNP. The idea that Scotland’s £13bn deficit can be halved within a few years without austerity is absurd. That would either require unprecedented cuts to public services, higher taxes and more borrowing - or more likely a toxic mix of all three.

“The SNP should be focusing improving education standards, fixing our NHS and growing our economy - not obsessing about separation.”

Mr Mackay also said the SNP would want an independent Scotland to be in the EU, but said it was not his party’s policy to join the euro.

Pamela Nash, chief executive of the Scotland in Union group, added: “The SNP’s latest independence blueprint is a recipe for deeper and tougher austerity, which would inflict hardship on families across our country.

“Derek Mackay’s support for ditching the pound and introducing a new currency is the height of irresponsibility, and even he has been forced to admit it would be high-risk.

“He is all over the place, also astoundingly ignoring that new member states in the EU must commit to joining the euro.”

Scottish LibDem Leader Willie Rennie said: “Derek Mackay is making it up as he goes along.

"The Growth Commission already states that the cuts to public services would be deep for an independent Scotland.

"But that is not enough for the SNP - they want it faster and deeper. Brexit shows that breaking up long term economic partnerships is painful - independence would be so much worse.”