Prime Minister Theresa May is expected to reject Nicola Sturgeon's demand to hold a second referendum on Scottish independence during the current Parliament.

Scotland's First Minister confirmed she will seek formal approval next week from the Scottish Parliament for a public vote to be held between autumn 2018 and spring 2019.

ITV News Political Editor Robert Peston said Mrs May was hard pressed to block the move for another referendum altogether but is most likely to push for a deal to hold it after the Brexit process has been finalised.

"She will reject Nicola Sturgeon's demand for a referendum in the Parliament," Peston said. "She would like to reject it altogether but she's not sure that she can hold firm on that."

He added: "So I expect over the course of the next days and weeks she'll probably move towards an offer of a deal after the next election by which time, she will argue, Scotland will be in full possession of the knowledge of our future relationship with the EU and they'll have a better basis on which to make their vote."

Ms Sturgeon's announcement came hours before the prime minister received the green light from Parliament to trigger Article 50 and formally begin Britain's EU withdrawal at the end of this month.

The two-year formal exit process would thus stretch to spring 2019, the limit of Ms Sturgeon's preferred timeframe for the Scottish public to vote on whether to split from the UK.

But Peston said Mrs May will look to push the referendum beyond 2020, the latest point her government can seek re-election as its five-year term comes to end, so she can focus on Brexit talks and pressing domestic issues.

Nicola Sturgeon made her referendum announcement at Edinburgh's Bute House, where she hosted the prime minister in July last year. Credit: PA

"She has no capacity to fight that referendum in the course of this Parliament because she's got this astonishingly complex set of negotiations to take us out of the EU and she's got public services that many people think are in crisis," Peston said.

Former first minister Alex Salmond said his successor's surprise announcement showed how much the prime minister had "underrated Nicola Sturgeon".

Mr Salmond told ITV's Good Morning Britain that Mrs May's speech at the Scottish Conservative conference earlier this month that had propelled Ms Sturgeon to act.

Alex Salmond said Nicola Sturgeon was entitled to fulfil a manifesto pledge to move for a second referendum.

He said Mrs May's address suggested she wanted to "grab" all the power coming back from Europe after Brexit for Westminster, would not compromise with the demands from the SNP and had attacked the Scottish government's handling of public services.

"It was that speech that Nicola said, 'she's going to concede absolutely nothing despite the previous assurances, therefore I have to fulfil a manifesto commitment (when she was re-elected last year) to do exactly this'."

Mrs May strongly condemned Ms Sturgeon's announcement as "deeply regrettable" and said the move was setting Scotland on a course for "more uncertainty and division".

The Union, Saltire and EU flags fly outside the Scottish Parliament. Credit: PA

The prime minister said she would negotiate a Brexit deal which worked for "the whole of the UK - that includes the Scottish people" and pointedly added: "Politics is not a game."

The prime minister's hope of securing a free trade agreement within the two-year Brexit deadline has been described as "out of the question" by former European Commission vice president Viviane Reding.

Mrs May wants to secure a trade deal alongside the UK's "divorce" settlement from Brussels during the two years allowed under Article 50.

The prolonging of Brexit negotiations could further muddy the argument on when a repeat Scottish vote could or should be held.