However, shortly before a deadline for amendments to be tabled at the Commons report stage yesterday, the UK government was forced to admit it would not keep its commitment, catching its own MPs off guard.
The failure was also condemned by Mr Mundell’s party colleagues at Holyrood, with Scottish Conservative constitution spokesman Adam Tomkins saying he was “deeply frustrated and disappointed”. SNP Europe spokesman Stephen Gethins said Mr Mundell was “guilty of selling out Scotland”.
Last night SNP and Labour MPs called on Mr Mundell to come before Commons and correct the record.
Clause 11 of the EU (Withdrawal) Bill sets out how 111 powers currently held in Brussels will return to Westminster after Brexit – despite being in areas that are devolved under the Scotland Act.
The Scottish Government has said it cannot accept the bill as currently drafted, and will reject a legislative consent motion at Holyrood without significant changes.
Last night Mr Mundell said in a statement: “I regret that has not been possible to bring forward amendments at the report stage but our commitment to improve the bill remains absolute.
“The most important thing is that the changes we bring forward command support on all sides, and talks between Scotland’s two governments will continue.”
A UK government source said the resignation of former Cabinet Office minister Damian Green, who was leading Brexit talks with devolved administrations, had caused the timetable to slip.
It is understood that Mr Mundell spoke with the Scottish Government’s Brexit minister Michael Russell by phone early yesterday to inform him of the delay. Mr Green’s replacement, David Lidington, contacted Deputy First Minister John Swinney yesterday to restart talks between the two governments.
With the SNP not naming peers to the House of Lords, the party will not have a direct role in amending the bill in the second chamber. However, it is understood that a senior crossbench peer will act on the party’s behalf, as was the case when the Scotland Act was debated in the House of Lords.
Scottish MPs of all parties issued a joint call for amendments to be made in the Commons in a report by the Scottish affairs committee last year. Chairman Pete Wishart said it was “totally unacceptable” for changes to be left to unelected peers.
In November, The Scotsman revealed that talks had been held between senior SNP figures and peers as a “fallback” in case amendments were not secured by the time the legislation left the Commons.
At the time, Mr Tomkins suggested changes would be made in the Lords, before Mr Mundell raised the prospect of an accelerated timetable. The delay could cause problems for the government when it comes to the final vote on the bill in the Commons later this month, after Scottish Tory MPs rejected Opposition changes, on the understanding that the government would bring forward its own amendments.
During debate on the bill at the committee stage in December, Scottish Tory MP Paul Masterton called for “quick progress” on amendments, adding that they should be made “certainly before the debate on report”.
Challenged by the SNP’s Tommy Sheppard in the Commons two days later over when changes would be made, Mr Mundell said: “It will happen on report. We have been very clear about this.”
Referring to Mr Masterton’s demand, the Scottish Secretary added: “The committee stage is about listening and adapting to issues that have been raised; we have listened to my honourable friend, the Member for East Renfrewshire, and we will table amendments to clause 11.”
Shadow Scottish secretary Lesley Laird said amending the bill in the Lords “shows nothing but contempt for democracy”.
Ms Laird said: “The Scottish Tories apparently voted for the bill based on the false promise that it would be amended at the next stage.
“It is time for Ruth Davidson and the Scottish Tory MPs to do the right thing and condemn the bill in its current form as it fails their key test, namely to protect the Scottish devolution settlement.”
Liberal Democrat Scotland spokeswoman Christine Jardine said Scottish Tory MPs were “disrespecting the wishes of their constituents who voted overwhelmingly to remain in the EU” if they supported the unamended bill when it returns to the Commons next week.
Mr Masterton said the delay was “disappointing” and warned the UK government would not secure legislative consent without amending the bill, but said he was still prepared to vote for the legislation as long as ministers do not change course.