The Scottish parliament's new tax and welfare powers "just won't work" unless Westminster and Holyrood fully cooperate.
That's the warning from the man who brought Scotland's political parties together to hammer out the powers blueprint after the independence referendum 'No' vote.
Lord Smith of Kelvin told ITV Border's Representing Border that Brexit had "put a spanner in the works" in terms of relations between the two governments but they still had to cooperate.
After the referendum then Prime Minister David Cameron asked Borders resident Lord (Robert) Smith to get agreement between the Tories, Labour, the SNP, Greens and Lib Dems on new powers after the 'vow' by the Unionist parties to give MSPs more power.
In an interview for a special programme which looks at his career - from growing up in Glasgow's Maryhill to senior roles in business and chairing the company which ran Scotland's Commonwealth Games - Lord Smith also reveals 'high-ranking' politicians tried to influence the work of what came known as The Smith Commission.
I asked Lord Smith why relations between the two governments were so important to him, and for the country? His Smith Commission report emphasised the need for cross-border co-operation:
We also asked the Scottish and UK Government's to respond to Lord's Smith's call. They agree in principle that they should world closely together, but the politics of Brexit loom large in their responses.
SNP Deputy First Minister, John Swinney, who was a member of the Commission told me:
Scottish Secretary David Mundell, who has been conducting negotiations with the Scottish government sees it a bit differently. He said:
South of Scotland MP, Mr Mundell added:
So there is hope then that Edinburgh and London can allay Lord Smith's concern. The big test of that will be when Theresa May next meets Nicola Sturgeon, which is scheduled to happen within the next few weeks.
There is also another intriguing aspect of Lord Smith's interview.
He told me that all five parties - the SNP, the Tories, Labour, the Lib Dems and the Greens - nominated two members each, who he described as "really good people."
Lord Smith continued:
I suggested that he was not going to tell me who those calls were from. His reply:
The Representing Border special programme featuring Peter MacMahon's interview with Lord Smith of Kelvin will be shown tonight on ITV Border in Scotland at 22.45.