HOLYROOD'S Constitution Secretary has urged rival parties to put aside their differences and join the Scottish Government in opposing the UK's "lunatic" Brexit plans.

Mike Russell made the plea as he told MSPs the latest round of negotiations with the EU had begun without agreement from the devolved administrations over their involvement.

He said this was "contrary to the devolution settlement" – noting devolved areas such as agriculture, fishing and the environment were among those being discussed in this latest stage of Brexit talks.

On Monday, Boris Johnson's chief Europe negotiator David Frost sat down with his counterpart from the EU, Michel Barnier, as talks began to strike a post-Brexit trade deal.

But Russell said: "We reject the published mandate as it is.

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"We will make clear that the UK Government, if it attempts to speak on matters that are devolved, does not speak for us."

In a statement to MSPs at Holyrood, he said: "The Scottish Government does not believe that Boris Johnson has any mandate, in any part of the UK, for a form of Brexit which was regarded as being on the lunatic fringe of politics, even during the June 2016 referendum.

"That form of Brexit, which the UK now regards as optimum, is a Canada-minus deal, the most basic of free trade agreements.

"Undoubtedly this will mean new barriers and borders, trade inhibiting rules of origin, customs difficulties and heavy regulatory requirements."

This approach, Russell argued, will severely impact "many of Scotland's most important sectors", such as seafood and farming.

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With the UK now in "an even more difficult phase of the Brexit process", he called on ministers in London to "move back from their aggressive rhetoric and ideological obsession with delivering a very hard Brexit".

Russell added: "I also urge this chamber to speak up for Scotland and to put differences aside to do so.

"The time and the threat demand that response from all of us."

Tory constitution spokesman Murdo Fraser told the Cabinet Secretary: "When we are dealing with what are very serious matters, throwing around ludicrous hyperbole, referring to the 'lunatic fringe', does him no favours at all.

"And the irony of this Government referring to others as ideological extremists will not be lost on many observers."