NICOLA Sturgeon has dramatically threatened to derail the legislation needed to put Brexit into effect, saying it appeared to be a “power grab” that would strip Holyrood of its authority.

The First Minister said the Great Repeal Bill would see powers which should be repatriated from Brussels to Edinburgh being intercepted and hoarded by Westminster.

In the latest constitutional clash between the Scottish and UK governments, Ms Sturgeon warned MSPs might withhold consent for the Bill, disrupting and delaying the process, and potentially forcing Westminster to overrule Holyrood for the first time since devolution.

It coincided with her signing the letter she will today send to Theresa May asking for a second independence referendum.

The UK Government said the Repeal Bill would lead to a “significant” increase in Holyrood’s total powers, and no current powers would be removed.

However there is now a fierce dispute over which devolved powers returned from the EU go to Holyrood after Brexit, with Ms Sturgeon demanding every last one of them, and Downing Street arguing it would make more sense to share some across the UK.

At First Minister’s Questions, Ms Sturgeon said: “I suspect that the Tories are planning a power grab on parliament. That will be absolutely unacceptable.”

Asked if the SNP Government would try to thwart the Bill by withholding consent in Scotland, Ms Sturgeon’s official spokesman said later: “In terms of removing competencies, we wouldn’t be giving legislative consent.

“We are not in the business of facilitating powers being stripped away from this parliament.”

The row broke out after Brexit Secretary David Davis published a White Paper outlining the Bill, one of the most complex pieces of UK law in decades.

Scottish Secretary David Mundell recently said he anticipated the Bill would mean Holyrood passing a Legislative Consent Motion [LCM] as it affects devolved areas.

But Mr Davis appeared confused when asked about an LCM in the Commons.

“At this stage we do not know, because we do not know the final format of the Bill. That is the simple truth,” he told SNP MP Joanna Cherry QC to her astonishment.

However Mr Davis’s own White Paper said the Bill would change the power of Scottish ministers, a textbook trigger for an LCM, giving Holyrood a crucial say in the matter.

Westminster could in theory ignore Holyrood and press ahead regardless, but this would be seized upon by the SNP to further its case for independence.

The Bill therefore gives Ms Sturgeon potential leverage in her pursuit of a referendum.

Brexit will see all powers exercised in Brussels on the UK’s behalf returned to Westminster in the first instance, and then some redistributed to Belfast, Cardiff and Edinburgh.

The SNP insists powers in devolved areas must go to Holyrood, and anything less would breach the founding text of devolution, the 1998 Scotland Act, which said powers are automatically devolved unless specifically reserved to Westminster.

However the White Paper said this would not happen, as there was a need to ensure “the effective functioning of the UK single market”.

Instead, some devolved powers such as agriculture, fishing and the environment, could be shared in “common UK frameworks”, rather than return wholesale to devolved administrations in a patchwork of different rules and laws.

It said: “Our guiding principle will be to ensure that no new barriers to living and doing business within our own Union are created as we leave the EU."

The farming and fishing industry broadly agree this is the most practical approach.

However the SNP is arguing for 100 per cent control in every devolved area.

SNP Brexit Minister Michael Russell said the White Paper suggested Westminster planned to commandeer powers that rightly belonged at Holyrood.

He said: “For the UK government to seek to impose legislative frameworks on these areas would be to take the unprecedented step of extending its powers over Scotland and must not take place. "The Scottish Parliament’s competences must not be diminished as a result of Brexit."

He was backed by the pro-independence Greens but also by Scottish Labour.

Scottish Green co-convener Patrick Harvie accused the UK Government of using Brexit to “hoard” powers that ought to go to Holyrood.

He said: “The UK government has already failed to deliver its promise to entrench the permanence of the Scottish Parliament; it would add insult to injury if the UK government use Brexit as an excuse to undermine the powers exercised here.”

Scottish Labour MP Ian Murray added: “All repatriated powers in devolved areas that return to the UK following Brexit should, in principle, fall under the competency of the devolved administrations. However, this will be a complex process with a number of different stages.

“Both the UK and Scottish governments have a responsibility to protect the interests of those affected, including businesses, and our farming and fishing industries.”

Besides repealing the 1972 European Communities Act that took the UK into Europe, the Bill will transpose most EU law into UK law so there are no “gaps” in the law at Brexit.

Once out of the EU, this body of transposed EU law can then be repealed piece by piece, although most of it is expected to be retained as useful and desirable.

To help the legislative clean-up operation, the GRB will give UK and devolved ministers sweeping “Henry VIII powers” to let them amend acts of parliament with minimal scrutiny.

Westminster sources pointed out that if Holyrood delayed the Bill, it could generate problems in Scots law and so prove self-defeating.

Scottish Tory Adam Tomkins said: “The SNP is complaining about the return of substantial new powers which - under its plans - would remain in Brussels.

“If ever people needed to see their utter hypocrisy, this is it.

"As the Prime Minister has made crystal clear, the UK Government expects substantial new powers to be delivered to our devolved parliaments as a result of Brexit.

“At the same time, it is sensible to ensure that UK frameworks are applied so that, as those powers come back, nothing is done that fragments our own United Kingdom.

“That might be good for the SNP but it would be bad for Scottish jobs."

Mrs May's spokesman brushed aside Ms Sturgeon’s concerns about a “power grab”, stressing no current Holyrood powers would go and “significant new powers” would be added.

Asked if there was concern in Whitehall that Holyrood could derail the Brexit process, he said: “This is a two-year process and we are confident it will be completed in two years.”