Speaking to party activists, the Tory leader said: "It is now clear that using Brexit as the pretext to engineer a second independence referendum has been the SNP's sole objective ever since last June".
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".
May had previously said there was no appetite for a second referendum less than three years after Scots voted by 55% to 45% to reject independence, in September 2014.
"If the prime minister refuses to engage on the terms of the referendum before Brexit takes place, then she is effectively trying to block the people of Scotland having a choice over their future".
Responding on Twitter, Sturgeon said denying a second referendum would be "undemocratic" and that her Scottish National Party (SNP) has a clear mandate to call a new vote.
"The status quo has been overtaken by events because unless powers now with the European Union are repatriated from Brussels to the Scottish Parliament, the Welsh Assembly and the regions, Whitewall will have perpetrated one of the biggest power grabs by further centralising power", Mr Brown is expected to say.
Scottish First Minister Nicola Sturgeon announced on Monday that she wanted a fresh vote on leaving Britain, saying Scotland did not want the "hard Brexit" that May's Conservative government is pursuing.
The problem is that Theresa May, and her Scottish Secretary David Mundell, have both said they won't even discuss the issue.
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That would then see the Scottish Parliament formally request that Westminster grants it the authority to hold a legally binding referendum.
"The Scottish Parliament has no power to amend or qualify reserved matters".
His comment comes amid a brewing constitutional crisis over Theresa May's decision to dismiss Nicola Sturgeon's demand for a new Scottish referendum until after Brexit.
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". Though the compulsions are different, the economic factor is the common grouse against the European Union and United Kingdom. of both Britain and Scotland.
"It becomes the will of the democratically elected Parliament of Scotland". "It is time for the First Minister to act responsibly over our future".
But he said that, with Mrs May due to invoke Article 50 by the end of this month, no agreement had yet been reached.
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.
"Just at this point, all our energies should be focused on our negotiations with the European Union about our future relationship".