HOLYROOD faces being bypassed by Theresa May on the crucial final Brexit Bill that will put the agreed UK-EU deal into law.

The SNP leadership said such a prospect was constitutional "madness".

The UK Government intends to introduce a series of bills relating to Britain's withdrawal, including the current legislation, the EU Withdrawal Bill, which will transpose all EU law into domestic law. There will also be a number of other bills, on some of which, such as those covering agriculture and fishing, the UK Government will seek the consent of MSPs.

However, there is now a doubt as to whether or not the Prime Minister will seek agreement from the Scottish Parliament on the last but most important piece of Brexit legislation: the Withdrawal Agreement and Implementation Bill, which would put any final deal into UK law.

When asked if the UK Government would seek MSPs' consent on this bill, a spokesman for the Department for Exiting the EU said: "The Government has announced its intention to bring forward a Withdrawal Agreement and Implementation Bill to implement the withdrawal agreement in domestic law.

"As with all new legislation we will continue to work closely with the devolved administrations in preparing for a smooth and orderly exit."

He then added: "The Government fully respects the Sewel Convention. As with all legislation, once the bill is further developed, the Government will consider whether the Sewel Convention applies." The convention involves Westminster seeking the consent of Holyrood if it wants to legislate on devolved matters.

Ian Blackford, the SNP leader at Westminster, was dismayed by Whitehall's response, suggesting it raised the possibility that Mrs May would seek to bypass Holyrood.

"This will be the most significant Act of Parliament on Brexit and will affect the whole of the UK.

"It would be madness for the UK Government to go down the route of not allowing MSPs to have a say on the most important piece of Brexit legislation."

The Highland MP added: "The department's response is quite remarkable and shows Westminster does not get how devolution works. It illustrates a lack of respect for the devolved administrations."

It is thought that one possible line of argument as to why the PM would not seek the consent of MSPs, or indeed AMs at the Welsh Assembly, on the final piece of Brexit legislation is that the Implementation Bill would in effect be putting into law a new treaty and foreign affairs are reserved to Westminster.

However, given the battle Whitehall is currently having with Edinburgh and Cardiff over the Withdrawal Bill, it would be nothing compared to the constitutional row created if it did not seek to get the consent of MSPs and AMs on the final key piece of Brexit legislation.

It could be that Mrs May and her colleagues know that with regard to the Implementation Bill, Holyrood would not give its consent in any case; a bill that put into law Britain's departure from the European single market and customs union would be anathema to Nicola Sturgeon and her party.

Meanwhile, politicians on either side of the border are preparing for what has been dubbed “Super Wednesday”; at Westminster, peers will debate Clause 11 of the UK Government’s flagship EU Withdrawal Bill and its amendment that seeks to allay fears of a power-grab – so far unsuccessfully – while at Holyrood, MSPs are expected to pass the Scottish Government’s Continuity Bill, which seeks to protect the devolved settlement in the event of no agreement on the UK bill.

Earlier this week, Nicola Sturgeon emerged from Downing St talks with Theresa May in an unexpectedly upbeat mood, suggesting a deal on the Brexit legislation could be done; she said while her differences with the Prime Minister were not insignificant, nor were they insurmountable.

However, Whitehall sources have suggested the First Minister’s optimistic mood was more due to her not wanting to rock the political boat on the day of Mrs May’s keynote statement on the Russia crisis.

If, as is expected, MSPs back the Continuity Bill, then UK Government lawyers will have six weeks before Royal Assent to consider whether to mount a legal challenge, which would stop the Holyrood legislation in its tracks.

The deadline for the Conservative administration to get the consent of MSPs will come after the sixth and final day of the Withdrawal Bill’s Report Stage in the Lords; expected to be in early May.

If at this stage the Scottish Parliament is still minded to withhold consent, The Herald understands the PM and her colleagues would be determined to press on with what they regard as a “vital piece of legislation”.

At the same time, Whitehall lawyers would go to the UK Supreme Court, the arbiter of devolution issues, to block the Continuity Bill.

Such a conjunction of events would plunge Britain into a constitutional crisis.