This morning the Prime Minister will pledge to forge a "more united" nation as she begins the Brexit process.
The prime minister was speaking in Cardiff as she tours Wales, Scotland and Northern Ireland in an attempt to create a Brexit consensus ahead of Article 50.
At a briefing to journalists on Friday, a Downing Street spokesman said: "It [calling a referendum] is a reserved power and the PM has been clear that now is not the right time".
Ms Sturgeon will say: "Whatever our different opinions on independence, we can all unite around this simple principle - Scotland's future must be Scotland's choice". Arguably, there could be substance in Ms Sturgeon's cavil that London had failed to address Scotland's desire to remain in the single market after Britain leaves.
Scottish nationalists have called for a new referendum and accuse May of all but ignoring their demands in her preparations for divorce talks with the European Union.
The impassioned speech, which received a standing ovation from the hundreds of Tory activists in attendance, comes just after Sturgeon accused May of "undemocratic" behaviour.
Politically, it may not be easy for Ms May to refuse another Scottish referendum for which the two Parliaments ~ in Britain and Scotland ~ will have to extend their approval.
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Speaking to ITV at the Scottish National party's spring conference, Sturgeon said she was focused only on winning a mandate from Holyrood next week authorising her to ask May formally for the authority to stage an official referendum.
"That means building a stronger economy and investing in the things that will deliver for Britain in the long-term, tackling the problem of low productivity and helping to secure the high-paid, high-skilled jobs of the future", she is expected to say.
First Minister Nicola Sturgeon has refused to rule out holding a wildcat independence referendum unauthorised by Downing Street.
When the prime minister rejects that request, Sturgeon will accuse May of undermining Scottish sovereignty on behalf of a government with no mandate in Scotland, in the hope of doubling popular support for a new referendum to force May into a U-turn. But under Britain's constitutional arrangements, that vote would have to be signed off by the British parliament. "May's announcement corresponded with pundits" predictions that a second Scottish independence vote would turn on a question of timing.
Under the terms of the European Union treaty, Britain will no longer be a member of the bloc two years after May issues the notification.
Meanwhile, former Scottish first minister, Alex Salmond, warned that the Scottish Nationalists could abandon their previous proposals for maintaining a currency union with the United Kingdom, in an interview with United Kingdom broadsheet the Financial Times on Friday.
Mrs May said there would be "further discussions" with devolved administrations during the Brexit negotiations.