The Scottish Parliament has the power to hold an independence referendum without Westminster’s consent, according to a previously secret UK government email revealed on Tuesday.

Correspondence released by the National Archives shows a message written by Tony Blair’s special adviser on Scotland just hours before his Government released its devolution plans.

In the email, Pat McFadden said that Donald Dewar – the man who went on to serve as leader of Scottish Labour and the country’s inaugural First Minister – took the view that a Scottish Parliament could “have a referendum on whatever it likes”, even on matters outside its legislative competence.

The exchanges are likely to renew fears among Unionists that the SNP could attempt to push through its own wildcat independence referendum bill if Boris Johnson continues to block a second vote. 

While Nicola Sturgeon has insisted that it would be a “dereliction of duty” to hold an independence referendum while the Covid-19 crisis is ongoing, her manifesto pledges to stage a vote before the end of 2023.

In an email to Mr Blair’s chief of staff Jonathan Powell in July 1997, Mr McFadden said some Scottish MPs were alarmed “such a referendum could take place” and “about the slippery slope to independence”.

Mr McFadden also asked Mr Powel to find out Mr Blair’s view on the issue as a matter of “urgency” – to which there is no reply on the government archive.

In an email sent two days before Mr Dewar, then Scottish Secretary, published the UK government’s White Paper on devolution, Mr McFadden said weekend newspapers had contained “some speculation over whether the Scottish Parliament would be able to hold a referendum on independence” – what the SNP had dubbed a “glass ceiling”.

Mr McFadden said there was obviously a glass ceiling to a Scottish Parliament because Westminster stayed sovereign.

“But leave that aside for the moment,” he said. “The reserved powers model means that the Scottish Parliament will have the power to legislate on anything not in the reserved list.

“Therefore it can have referendums on anything it wants, even if it cannot enact the result.

“A couple of very worried Scottish MPs have rung me about this. It scares them a great deal that such a referendum could take place.

“Donald’s view is that the Scottish Parliament can have a referendum on whatever it likes, even matters outside its competence, which is in line with the logic of the White Paper.”

Poll: Support for Scottish independence in England

However, Mr Dewar appeared to change his mind when the Scotland bill was debated the following day, telling MPs: “It is clear that constitutional change – the political bones of the parliamentary system and any alteration to that system – is a reserved matter. 

“That would obviously include any change or any preparations for change. A referendum that purported to pave the way for something that was ultra vires is itself ultra vires.”

The debate over the legality of Holyrood holding its own referendum is likely to intensify in the coming months as the country recovers from the pandemic, with some legal scholars arguing that it can hold a referendum to test public opinion on the Union even if it can’t legislate on the matter.

If Westminster continues to oppose a second poll, SNP policy is to push through its own bill and “vigorously defend” it in court – effectively daring UK ministers to take legal action to block it.

It comes after reports last month that Dorothy Bain QC, Scotland’s new Lord Advocate, would be “likely to give a referendum bill the green light” once installed as the Scottish government’s most senior legal adviser.