The centrepiece of the deal is a £1.5bn package of public spending for Northern Ireland, £1bn of it new money, in return for DUP support for May’s minority Tory government in parliamentary votes of confidence or supply.
No 10 insisted that the funding was additional to the Northern Irish block grant and therefore not subject to the Barnett Formula, the system for apportioning resources between the constituent nations of the UK. A parallel was drawn with money for city deals.
But Ian Blackford, the Scottish National Party’s Westminster leader, said the “grubby” deal showed “how little the Tories care for Scotland,” and how little authority, influence or credibility Ruth Davidson’s new Scottish Conservative MPs exercised within their party.
Labour’s Scottish leader Kezia Dugdale warned May against “weakening the bonds that unite the UK, and the Greens’ Patrick Harvie called the deal “shameless.”
There was similar anger in Wales, which also looks to the Barnett Formula for a share of any new public spending sanctioned by Whitehall. First Minister Carwyn Jones said the deal with the DUP "further weakens the UK" and “all but kills the idea of fair funding for the nations and regions”.
Immediate pressure fell on Scottish secretary David Mundell who said in a BBC interview a week ago that he would block any deal with the DUP that “deliberately sought to subvert” Barnett. He was also quoted in the weekend press as promising to resist “back door funding” for Northern Ireland.