THERESA May will be seen by voters as scorning the Scottish Parliament and undermining devolution if she refuses to allow a second referendum, Nicola Sturgeon has warned.

As she formally asked for Holyrood to be granted the power to hold another independence vote, the First Minister told Mrs May she had “no rational reason” to deny the request.

MSPs voted on Tuesday by 69 to 59 to seek the relevant transfer of power from Westminster, after the three Unionist parties were outnumbered by the SNP and Scottish Greens.

The Prime Minister has already said “now is not the time” for a new referendum, given the UK Government’s intense focus on Brexit negotiations over the next two years.

The difficulty of the Brexit process was underlined yesterday when draft EU guidelines ruled out early free trade talks, and said no agreement on the EU's future relationship with the UK would apply to Gibraltar without the consent of Spain, handing Madrid a potential veto.

European Council president Donald Tusk said the EU would not "pursue a punitive approach", but predicted the talks would still be "difficult, complex and sometimes even confrontational".

But in a letter emailed to Downing Street, Ms Sturgeon said the people of Scotland did not vote for Brexit or to be taken out of the EU single market, as Mrs May now proposed.

She wrote: “In these very changed circumstances, the people of Scotland must have the right to choose our own future - in short, to exercise our right of self determination.

“In anticipation of your refusal to enter into discussions at this stage, it is important for me to be clear about my position.

“It is my firm view that the mandate of the Scottish Parliament must be respected and progressed. The question is not if, but how.”

The First Minister said she felt frustrated that the voices of Holyrood and the other devolved administrations had been “largely ignored and all attempts at compromise rejected” ahead of the UK government triggering Article 50 on Wednesday.

She claimed the Prime Minister’s timetable for divorce terms and an EU trade deal being known by late 2018 matched her own schedule for a referendum between autumn 2018 and spring 2019, allowing Scots a choice between Brexit in the UK and independence.

Asking for “early discussions” on transferring referendum powers to Holyrood under Section 30 of the 1998 Scotland Act, she said the precedent of the Edinburgh Agreement, which underpinned the 2014 referendum, should make agreement “relatively straightforward”.

However Ms Sturgeon's request was met with a swift refusal from the UK Government.

A spokeswoman said: "The Prime Minister has been clear that now is not the time for a second independence referendum and we will not be entering into negotiations on the Scottish Government's proposal.

"At this point, all our focus should be on our negotiations with the European Union, making sure we get the right deal for the whole of the UK.

"It would be unfair to the people of Scotland to ask them to make a crucial decision without the necessary information about our future relationship with Europe or what an independent Scotland would look like.”

As well as her letter, the First Minister also posted a video on social media in which she said: "The Prime Minister has indicated that she intends to ignore the will of the Scottish Parliament and seek to prevent people in Scotland having that choice. If the Westminster government continues to hold that line, it will go against the very foundations of devolution.”

She said she hoped Mrs May would change her mind, but if she didn’t, she said she would update Holyrood "on how we're going to move forward to ensure that the people of Scotland are able to choose our future” when it returned from Easter recess.

Government sources said the Brexit situation was so fluid and fast-moving, Ms Sturgeon had not chosen which steps to take next, but the overarching goal was to sway public opinion.

An SNP-commissioned poll this week found most Scots want Holyrood, not Westminster, to decide the timing of any referendum, although there is still little appetite for one before Brexit.

The Scottish Conservatives dismissed Ms Sturgeon’s request as “unwanted and workable”.

Deputy leader Jackson Carlaw said: "At the very moment we should be uniting as a country to get the good deal out of Brexit, Nicola Sturgeon is trying to divide us again.

"She wants a referendum campaign to start right now - despite still not having answered basic questions on the currency, on EU membership and the cost of independence.

"We have had enough of SNP stunts and First Ministerial photo-calls. Nicola Sturgeon should dump her referendum plans now, work to get the best deal possible Brexit deal for Scotland and the UK - and then get back to the day job of improving our schools and hospitals."

Scottish Labour leader Kezia Dugdale said people would be “dismayed that Nicola Sturgeon is more focused on rerunning the arguments of the past than doing her job”.

She said: “There is absolutely no evidence that another divisive referendum is the will of the people of Scotland. We need the SNP government to get on with the job of governing.

“That means focusing on jobs, growing the economy and helping family incomes.

"Holding another referendum is the wrong thing to do for Scotland’s economy, especially when there is so much economic uncertainty from the Tories’ reckless plans for a hard Brexit.”

Scottish LibDem leader Willie Rennie said the SNP were “shameless” in using Brexit as a pretext to advance independence

He said: “The SNP are refusing to guarantee that an independent Scotland would be a full member of the EU meaning we are facing a prospect of Scotland out of both single markets.

"It is a Government obsessed with constitutional issues rather than addressing the day job."