UK ministers have until Tuesday May 8 to agree a deal with their Scottish counterparts on the flagship EU Withdrawal Bill or face a major constitutional crisis.

This is the sixth and final day of the legislation’s Report Stage in the House of Lords when peers will vote on any proposed changes. The bill will then go to Edinburgh to get the consent or otherwise of MSPs.

While the UK Government has maintained its confidence that Nicola Sturgeon and her colleagues will support the bill, there is little tangible evidence yet that this is the case.

The First Minister as well as her Welsh counterpart Carwyn Jones believe that the UK bill is a “power-grab” by Whitehall as Theresa May wants a temporary hold on some 24 powers and responsibilities going to Holyrood from Brussels post Brexit. This, the Prime Minister argues, is to ensure that there are countrywide common frameworks covering issues like agriculture, fisheries and environmental protection so that the UK’s internal market is not harmed.

But Ms Sturgeon has insisted the principle of devolution must be defended and that all 100-plus powers and responsibilities that leave Brussels on Brexit Day must go straight to Edinburgh.

The SNP has no representation in the Lords but it is known that many peers are unhappy at the prospect of Edinburgh and Cardiff not giving their consent to the legislation; there have even been claims the second chamber could seek to block the Withdrawal Bill on May 8 in the knowledge that it does not have the support of the devolved administration and parliaments.

Mrs May and her Tory Government do not have a majority in the Lords.

Whitehall insiders, who regard the legislation as “vital” to the successful process of a smooth Brexit, have made clear that in the absence of consent from MSPs and AMs, the UK Government will press ahead with the bill.

As was emphasised during the Article 50 court case approval from Holyrood through a so-called Legislative Consent Motion is purely a constitutional convention. Yet the political backlash to pushing through the bill against the wishes of the Scottish Parliament and the Welsh Assembly is likely to be great.

To add to the constitutional tensions, UK Government lawyers, in a parallel process, are preparing to mount a legal challenge to the devolved parliaments’ own Brexit legislation, the Continuity Bills. The deadline for this is earlier: April 18; two days after Westminster returns from its Easter recess.