THERESA May must use her final week in power to revoke Article 50, the Scottish Greens have warned.

Either Boris Johnson or Jeremy Hunt will take up the post of prime minister next week, and MSP Andy Wightman urged May to act to stop a no-deal Brexit.

The Greens politician described Johnson, the clear frontrunner in the Tory leadership contest, as a "dangerous fantasist".

Wightman was among the "Scottish Six" who defeated the UK Government in a legal case which saw the European Court of Justice rule that Britain can unilaterally stop the Article 50 process.

He was joined in that effort by colleague Ross Greer, the SNP's Joanna Cherry QC MP and Alyn Smith MEP, Labour MEPs David Martin and Catherine Stihler, and English lawyer Jolyon Maugham QC.

Wightman said: “It has become clearer than ever that Boris Johnson is prepared to lead the UK into a catastrophic event that even his own government admits would damage the economy, place imports and exports on punitive tariffs, impact the medicine supply chain and create a hard border in Ireland.

“The power over our future is about to placed into the hands of a dangerous fantasist, more interested in his own career than the livelihoods of the citizens of the UK. 

“Thanks to the legal case which I led to the Court of Justice of the European Union, which ironically Theresa May tried to thwart, she now has the power to put an end to this and bring Brexit to a halt.

“She has just days left to see sense and revoke Article 50 before she hands the reins over to someone who clearly doesn’t have our best interests at heart.”

Johnson has made a "do or die" commitment to leave the EU by the Halloween deadline.

He has even toyed with the controversial idea of suspending Parliament to prevent MPs taking steps to prevent a no-deal exit.

Hunt has called October 31 a "fake deadline", and said he would be willing to extend the due date if a deal with the EU was within reach.

The decision on whether to end talks with Brussels would come at the end of September, Hunt said.

They have also clashed on the so-called Gatt 24 mechanism, which Johnson has said could be used to avoid tariffs in the event of a no-deal scenario.

Hunt said that this was "factually" not possible, adding that he would not say Johnson's claim was "a lie because that suggests someone is doing something deliberately".

But both agree that the best solution would be to broker another deal to Theresa May's and that, if it came to it, they would take the UK out of the EU without a deal in place.

The contenders must resolve a key sticking point to the outgoing Prime Minister's agreement – the Irish backstop to prevent a hard border on the island in the event of a No Deal.

Hunt is prepared to change or trash the current proposal and Johnson describes it as a "dead letter", or effectively defunct.

To prevent a hard border in the future, Hunt said there is "technology there now" and his rival suggested it would be worked out with a free trade deal during the implementation period.