NICOLA Sturgeon has warned that a "blatant power grab" by the Tories poses the biggest threat to the Scottish Parliament's powers since the vote for devolution two decades ago.

The First Minister's dramatic claim came ahead of a landmark speech she is to deliver tomorrow to mark 20 years since the historic devolution referendum on September 11, 1997.

Writing for the Sunday Herald, Sturgeon said the devolution settlement is now under unprecedented attack from the UK Government's Brexit plans to repatriate powers from Brussels to London.

She warned of the urgent need to defend the parliament in Edinburgh and to prevent a massive step backwards for Scotland's democracy.

She said the UK Government's EU withdrawal bill includes the "wholesale transfer of powers" from Brussels to Whitehall and Westminster.

However, she warned the transfer of those powers will see London take charge of policy areas traditionally devolved to Holyrood such as farming and fishing.

Sturgeon said the plan reverses the process of devolution for the first time since the opening of the Scottish Parliament in 1999.

She claimed that the move rips up the founding principles of the parliament as set out by the then First Minister Donald Dewar that all powers were to be transferred to Edinburgh other than those explicitly reserved to Westminster.

In today's Sunday Herald, Sturgeon writes: "The UK Government’s Brexit proposals – through the proposed EU (Withdrawal) Bill – constitute a blatant power grab which, far from enhancing the powers of the Scottish Parliament as has been claimed, would throw the process of devolution into reverse for the first time in two decades."

Sturgeon said the vote for devolution by Scots would be eroded by Tory plans that will see Westminster able to change the law using the powers repatriated from the EU, while Holyrood is left powerless in those policy areas.

She added: "That approach utterly demolishes the principle outlined by the late Donald Dewar in framing Holyrood’s remit, which made clear that all powers were to be transferred to Edinburgh other than those explicitly reserved to Westminster.

"The Tories’ Brexit proposals, by suggesting that devolved powers which are currently exercised at European level should be repatriated to London and not Scotland, ignore that principle.

"Indeed they breach it, meaning that the founding basis of the Scottish Parliament, endorsed in a nationwide democratic vote, would be eroded."

The First Minister also rejected claims by Tory ministers that powers taken from Brussels to London will eventually be transferred to the devolved administrations in Scotland, Wales and Northern Ireland.

Sturgeon warned that Theresa May's government will adopt an approach of "only some and only those which they saw fit to" over devolving any repatriated powers to Edinburgh, Cardiff and Belfast.

Ahead of her speech in Edinburgh tomorrow, Sturgeon talked about the "stark reality" facing Scotland two decades on since the start of devolution.

She also repeated her position that it is "inconceivable" that she will recommend that Holyrood gives its give legislative consent to the UK Government's withdrawal bill. Instead, Sturgeon calls for cross-party unity to protect the devolution settlement.

The SNP leader added that Carwyn Jones, the Labour First Minister of Wales, shares her view on the Tory plans.

Former Labour First Minister Henry McLeish, who presided over the introduction of devolution legislation as a Scotland Office minister under Donald Dewar at Westminster in the late 1990s, backed Sturgeon's call.

He called the Bill "one of the biggest abuses of power in the post-war period.

"It's a sad day for Scotland as this is a step backwards. It also shows that the Tories don't care about devolution."

However, a UK Government spokesperson, rejecting Sturgeon's claims, said: "The EU (Withdrawal) Bill will strengthen devolution. It is the opposite of a 'power grab'.

"The Scottish Parliament will lose none of the decision-making powers it currently exercises and we expect it will gain significant new powers."