The UK Government needs to "raise its game" and make talks with the devolved administrations over Brexit more effective, a committee of Lords has said
The Joint Ministerial Committee (European Negotiations) was set up to allow ministers from the UK, Scotland, Wales and Northern Ireland to discuss the process, but a new report said it "is clear that at a basic level its meetings are not being treated with respect or organised efficiently".
Peers on the House of Lords EU Committee insisted "this needs to change" as they said the committee should be allowed to agree common positions on the Brexit issues of most importance to the governments in Edinburgh, Cardiff and Belfast.
The call comes after Scottish Brexit Minister Mike Russell demanded the JMC meetings be urgently reconvened by the UK Government.
In their report, looking at the impact of exiting the European Union on the devolved administrations, peers said they back "the view of most of our witnesses that the UK Government needs to raise its game to make the JMC (EN) effective".
This should include better preparation, including bilateral discussions ahead of meetings, a structured work programme, greater transparency "and a willingness to accept that the JMC (EN), even if not a formal decision-making body, is more than a talking-shop", according to the report
Here the Lords suggested the JMC "should be authorised to agree common positions on key matters affecting devolved competences in time to inform the UK Government's negotiating position"
The report highlighted concerns over the "apparent deterioration of relations between the UK and Scottish governments" as a result of the UK's vote to leave the EU.
After almost two-thirds (62%) of Scots voted to remain in the 2016 referendum, the Lords also stressed there is a "strong political and economic case for making differentiated arrangements for Scotland".
However they dismissed Scottish Government calls to remain in the European single market after the rest of the UK leaves the trading bloc as being "politically impracticable, legally highly complex and economically potentially disruptive to the functioning of the UK single market".
The report made clear the "integrity and efficient operation of the UK single market must be an over-arching objective for the whole United Kingdom" during the Brexit process - but added "that objective does not preclude differentiated arrangements for Scotland in some areas, and nor does it justify excluding the Scottish Government from the Brexit process".
Ministers in London and Edinburgh need to "set aside their differences and work constructively together to protect the interests of the citizens of Scotland in the final Brexit deal", the Lords said.
While they made clear they did not want new areas of powers to be devolved, the report said the "specific labour market and demographic needs of the devolved nations should be accommodated in the context of Brexit".
With Scotland having relied on immigration to boost its population, the report said the UK Government should use its forthcoming Immigration Bill "to look for opportunities to enhance the role of the devolved institutions in managing EU migration", adding that "local and regional economic and demographic needs, rather than central targets, should drive decision-making".
In Northern Ireland it "appears that the Brexit debate has undermined political stability and exacerbated cross-community divisions", contributing to the collapse of the Northern Ireland Executive and the calling of an early Assembly election, the report said.
Consequently the Lords warned: "Political stability in Northern Ireland must not be allowed to become 'collateral damage' of Brexit."
Meanwhile, they said Wales could be "profoundly affected" by leaving the EU as the economy is "highly reliant" on membership of the single market - with the report saying it is "therefore particularly vulnerable" if UK access to this is reduced.
Lord Jay of Ewelme, a member of the committee and former head of the diplomatic service, said: "Brexit's impact on the future of the United Kingdom will be profound and unpredictable. At the moment the internal politics are pretty toxic, and we saw only last week the start of what could become a deep and bitter dispute on the role of the devolved institutions in passing the EU (Withdrawal) Bill.
"We can't afford this. The UK Government must respect the devolved institutions. It's not enough saying it's listening to them-it's actually got to take account of what they say and adjust its approach to accommodate their specific needs. Equally the devolved administrations must work with, not against, the UK Government to get the best Brexit for the whole of the UK."
Mr Russell said: "As is becoming clearer by the day, Brexit is causing deep uncertainty across Scotland - and an extreme Brexit outside the single market and customs union will cause severe long-term economic damage, hitting jobs, growth and living standards.
"This report makes clear Brexit should not be used as cover for a 'power grab', and that powers in areas such as fisheries, agriculture and environmental protection should be returning from Brussels to Edinburgh, Cardiff and Belfast - not Westminster.
"But just as there should be no power grab, there should be no attempt to use Brexit to cut Scotland's budget, and under the current constitutional arrangements the Scottish Government supports the retention of the Barnett Formula.
"This report also calls on the UK Government to 'raise its game' in consulting the devolved governments - UK ministers should heed that call by resuming the JMC process, as this report calls for, and by agreeing to involve the Scottish and other administrations directly with a seat at the negotiating table.
"Our clear position is that we want a way forward that allows Scotland to remain within the single market and this report notes the need for a specific Scottish solution. We will do everything we can to help deliver the best possible deal for our society and economy."
A UK Government spokesman said: "We have been clear that the Repeal Bill will not take away any decision-making powers from the devolved administrations immediately after exit.
"Instead, to protect the UK internal market, some decision-making powers being transferred into UK law will be held temporarily to allow intensive discussion and consultation with the devolved administrations.
"As the Secretary of State has made clear, it is our expectation that the outcome of this process will provide a significant increase in the decision-making power of each devolved administration and we are committed to positive and productive engagement."