THE escalating dispute over Brexit powers represents devolution's greatest ever crisis, the Scottish Trades Union Congress (STUC) leader has warned.

Grahame Smith said that hoarding powers at Westminster for traditionally devolved areas would be “an erosion of devolution”, and said the dispute had “exposed the centralising tendency of Westminster” towards the UK's devolved parliaments.

The STUC general secretary said it also showed how tenuous the devolution settlement was.

However, Smith said it represented the biggest threat to devolution since the Scottish Parliament opened in 1999.

He said: "The current dispute between the Scottish and UK Governments on the EU Withdrawal Bill has served to expose the more fundamental issue of the future governance of the UK post-Brexit.

"It has opened up questions about the nature of devolution in the UK and the role of devolved parliaments, questions that have been largely ignored in the two decades since devolution."

Smith said it would "not be acceptable" for Westminster to adopt the legislative role of Holyrood in any policy areas.

He added: "Whether or not one accepts that the EU’s decision-making processes are undemocratic, it would be unacceptable to replace them processes that are just that.

"The UK Government must accept the legitimacy of devolved institutions and realise that proposals which create a situation where the UK could legislate on any area of devolved competence without the agreement of the Scottish Parliament would be an erosion of devolution and would not be acceptable.

"The recent process has exposed the centralising tendency of Westminster.

"'Taking back control' seems to be a mantra limited to executive power in that jurisdiction alone rather than the expression of a desire to engage and empower communities the length and breadth of the country."