Theresa May is presiding over "constitutional chaos" across the UK on the day the Article 50 letter is signed, Scotland's Brexit Minister has warned.

Mike Russell said the Prime Minister is responsible for "a crisis in devolution" as she begins the formal process for exiting the European Union.

He described the moment as "sad and self-destructive" and the beginning of a "backwards journey" towards greater centralisation of power at Westminster.

Speaking at a debate in Glasgow hosted by The Sunday Times Scotland and law firm Brodies, Mr Russell said: "We're in constitutional chaos across these islands, there is no unanimity about this.

"We're here because the Prime Minister particularly will not compromise, will not debate and discuss the reality of the constitutional situation of these islands."

He insisted the Scottish Government would seek consent and dialogue over its plans for a second referendum on independence, after Holyrood on Tuesday voted in favour of seeking permission for a ballot to take place between autumn 2018 and spring 2019.

Scotland's First Minister Nicola Sturgeon has said her mandate for another vote is now "beyond question", but the UK Government has said it will decline any request for a section 30 order, the mechanism for the powers to hold a referendum.

Mr Russell said with the triggering of Article 50 "there will be a moment of choice between 18 months and two years' time", adding: "There will be a deal on the table and all we are saying is that is a deal that the people of Scotland should get to vote on."

He said the refusal of the UK Government to engage "for the first time ever" in discussion with the Scottish Government over the section 30 order is "a very serious situation".

He added: "That's the situation we appear to be in but it will only be resolved by negotiation and discussion. There simply isn't an alternative to that. Devolution works because there is consent and dialogue.

"We are now in a position where the Scottish Parliament has said this is what we want to happen and therefore we have to have dialogue. After the Easter recess if there is no progress we'll indicate further steps we will take.

"The situation in Scotland, in Northern Ireland, in Wales - we have a crisis in devolution and a crisis in the constitution across these islands and the central figure in that, regrettably, is the Prime Minister, therefore the Prime Minister needs to look at this, causing that constitutional chaos is immensely damaging for everyone."

Speaking at the same event, Scottish Conservative deputy leader Jackson Carlaw said: "What I need to see and what I think Scotland needs to see now is the Scottish Government actively working with the governments of Northern Ireland, Wales, the Channel Islands, Gibraltar and the rest of the UK to get the best possible deal.

"We can't afford a phoney war on something else while we should be concentrating on that task in hand and my concern is that having already said that they will campaign against whatever that deal turns out to be in favour of independence, they are undermining the confidence and trust that will need to exist between all the other parties in the UK to secure that future."