RELATIONS between the Scottish and Westminster parliaments have hit rock bottom because of Brexit, which is straining devolution to breaking point, MSPs warn today.

Holyrood’s Finance and Constitution Committee said there was a “clear impasse” over the operation and meaning of the Sewel Convention covering cross-border legislation.

It called for an urgent debate on whether devolution was “robust enough to deal with Brexit”, and questioned whether current, ad hoc, arrangements could survive.

The Committee issued its warning in a report on the UK Trade Bill, recommending that it should not get legislative consent from Holyrood as it impinged upon devolution.

The Bill relates to post-Brexit trade policy, some of which will affect devolved issues.

However it gives Scottish ministers only limited powers to deal with these, something the SNP government and most of the Committee said was unacceptable.

The Committee said it was “very disappointing” that the Bill was silent on the role of the devolved institutions in the negotiation of Post-Brexit trade arrangements.

It said the Scottish Parliament should therefore withhold legislative consent for the Bill in its current form, setting the scene for a constitutional clash with Westminster.

The issue caused a breakdown in a previous cross-party consensus on the 11-member Committee, with its three Tory MSPs refusing to back the withholding of consent.

However the Committee unanimously agreed there was an impasse over the Sewel Convention that needed to be addressed fast, and ideally before the Trade Bill is amended in the House of Lords.

The Sewel Convention says Westminster will not “normally” legislate in devolved areas without Holyrood’s consent, although ultimately it can do so.

Since devolution began in 1999, Westminster has only imposed legislation on Holyrood once - passing the EU Withdrawal Act this year despite most MSPs opposing it.

That pattern is now set to be repeated with other Brexit-related primary legislation, such as the Trade Bill, as the SNP government has said it will withhold consent on principle until the Sewel convention is changed to its satisfaction, and ideally embedded in law.

However the UK government has refused to make the Sewel convention legally binding, arguing that would give Holyrood a veto over Westminster in some cases.

Bruce Crawford, the SNP convener of the committee, said

he would write to Scottish Secretary David Mundell and SNP Constitutional Relations Secretary Miek Russell to request "they seek a solution to the impasse over the Sewel Convention as a matter of urgency".

Adam Tomkins, Tory deputy convener on the committee, said: “There is no good reason for the SNP to withhold legislative consent to the Trade Bill.

“The Sewel convention is not broken; indeed, the UK Government has recently repeated its full commitment to the Sewel convention and to ‘the related practices and procedures for seeking legislative consent’ and this is warmly to be welcomed.”