Nicola Sturgeon has challenged Theresa May to a debate about a second Scottish independence referendum, after No.10 vowed to block her request.

Asked if a “face-to-face debate” would be preferable to the two leaders staging press conferences and TV interviews, the First Minister replied: “I'd be up for it.”

The call came as Downing Street removed any doubt that the Prime Minister was ruling out a referendum before Brexit is completed, probably in early 2019.

“In that timeframe, as put forward by the First Minister, we will not agree to a referendum,” Ms May’s official spokesman said.

Asked about post-2019, he added: “We are not getting into future timeframes. We are responding to a specific request from the First Minister.”

Ms Sturgeon’s timeframe is for the referendum to be held between autumn 2018 and spring 2019 – after “the terms of Brexit are known”, but before withdrawal is completed.

The spokesman set out Ms May’s reason to make clear she will block the Section 30 request, which will be confirmed by the Scottish Parliament in a vote next Wednesday.

“We hope they might step back from next week’s vote now we’ve made our position clear,” he explained.

Earlier, Scottish Secretary David Mundell, went further, insisting the UK Government will not enter into any discussions with Edinburgh about holding a second referendum.

Speaking at a press conference, he said: “The proposal brought forward is not fair, people will not be able to make an informed choice. Neither is there public or political support for such a referendum.

“Therefore we will not be entering into discussions or negotiations about a Section 30 agreement and any request at this time will be declined.”

In her TV interview, Ms May said “now is not the time” for the referendum, because the country was fully engaged in the crucial task of winning the best Brexit deal.

"We should be working together to get that right deal for Scotland, that right deal for the UK. As I say that's my job as Prime Minister and so for that reason I say to the SNP: now is not the time,” she told ITV.

It was reported that the Conservatives had been preparing for this moment for three months, by testing the “no, not now” message with focus groups – and were confident it would work.

But Ms Sturgeon hit back immediately, saying: “It is for the Scottish parliament – not Downing Street – to determine the timing of a referendum and the decision of the Scottish parliament must be respected.

“Any bid by the UK Government to block the people of Scotland from making a choice will be untenable, undemocratic and totally unsustainable.”

The Scottish Greens confirmed they would give the SNP the majority they need at Holyrood to approve the referendum call, by also criticising Ms May.

Ross Greer, the party’s external affairs spokesman, said: “Scotland deserves to choose between the isolated, angry Brexit Britain planned by the Tories and putting our future in our own hands with independence.