BREXIT has shattered a key part of the SNP’s plan for leaving the UK, Alex Salmond’s former spindoctor has admitted.

Kevin Pringle, who was a senior special adviser to the former First Minister, said the promise to deliver independence just 18 months after a Yes vote was no longer credible.

It “doesn’t measure up” and a far longer transition period would be required, he said, suggesting this would need to be agreed “jointly” with Westminster.

Negotiating independence in 18 months was a key plank of the SNP Government’s White Paper on independence, launched by Mr Salmond and Nicola Sturgeon in 2013.

It stated that if Scotland voted Yes in September 2014, the negotiations would be wrapped up in time for Scotland to become independent on 24 March 2016.

The White Paper said this “realistic” date would allow time for deals with the UK Government and Brussels, including an agreement on Scotland becoming a member of the EU.

It said Scotland would be able to negotiate its share of UK government assets, the national debt, pension liabilities, and agree cross-border services on health, policing and security.

Scotland would also agree membership of the UN, Nato, the IMF, the World Bank, the World Trade Organisation, inherit a host of international treaties, and take control of welfare, defence, international development, citizenship, nationality and borders, and fiscal policy.

Mr Pringle, 51, said the lengthy process behind Brexit and delays in the limited devolution of welfare powers showed that timescale was clearly over-ambitious.

More than two and a half years have already elapsed since the UK voted to leave the EU, and many more years of negotiations are still needed on trade.

The SNP Government has also been forced to ask Whitehall for delays in the transfer of just 15 per cent of social security spending to Holyrood because of the complexity of the process, with changes legislated for in 2016 not taking effect until 2020.

Writing in the Sunday Times, Mr Pringle, who was Mr Salmond’s closest aide for many years, said experience had rendered the White Paper’s timetable defunct.

He said: “The proposed 18-month timescale for becoming independent after the September 2014 referendum doesn’t measure up, certainly in light of the Brexit negotiations and the time taken to transfer welfare powers to the Scottish Parliament.

“Perhaps it would be better to suggest a longer transition period, one jointly agreed by Holyrood and Westminster.”

The SNP’s recent revised economic blueprint for independence - which itself took 18 months to produce - did not give a timescale for delivering independence, so the 18 months in the White Paper remains the party’s only publicly declared position.

Pamela Nash, chief executive of anti-independence group Scotland in Union, said: “

"The Brexit process has made crystal clear that the 18-month timetable given by the SNP for Scotland to leave the UK was completely unrealistic.

"It’s now time for an apology from Nicola Sturgeon.

"The Brexit process has demonstrated how difficult it is to leave a union of nations. Scottish independence would be considerably more complex, and would be deeply damaging for our country."

An SNP spokesperson said: "We will fully and clearly set out plans for independence after the Brexit fog clears, and well ahead of any future referendum.

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