Liberal Democrat leader Tim Farron is to accuse the prime minister of following “aggressive, nationalistic” politics like those of Donald Trump and Vladimir Putin.
He will tell his party’s conference later that Theresa May is part of a “new world order” with the presidents.
His party is the “real opposition” to her “hard Brexit” plan, he will add.
Mrs May told the Conservative’s spring conference on Friday that she wanted a “more united” Britain.
Mr Farron will say he wants to reclaim the British flag from nationalists using it as a “symbol of division”.
He will make the comments as part of his keynote speech at the Lib Dem’s spring conference in York.
‘The new normal’
The party leader will say: “The politics of Trump. Of Putin. Of Le Pen. And now the politics of Her Majesty’s Government.
“Welcome to the new world order. This is the new normal, the new status quo.
“Aggressive. Nationalistic. Anti-Nato. Anti-EU. It is the post-war internationalist consensus unravelling in real time.
“Winston Churchill’s vision for a world that achieves peace through trade, common values and shared endeavour evaporating before our eyes.”
Mr Farron will tell his supporters to stop being “squeamish” about patriotism, calling for the centre left to reclaim the British flag.
“I love my country,” he will say. “I’m proud of my identity.
“Patriotism isn’t about dividing our society. It is about celebrating it. It’s about our shared experience, our shared history, our shared destiny.”
The Lib Dem leader will end his speech by saying his party, which has nine MPs, is the “only party in British politics opposed to a hard Brexit”, and he will continue to campaign for a referendum on the deal negotiated by the government on leaving the EU.
Meanwhile, former Prime Minister Sir John Major has criticised what he calls “ultra Brexiteers” within the Conservative Party who he believes are trying for a “complete break from Europe.”
Writing in the Mail on Sunday, he says such figures need to “stop shouting down anyone with an opposing view”, as well as launching “vitriolic and personal attacks” on judges, civil servants and other public figures.
“In doing so they demean both themselves and their cause,” he wrote. “These ‘Ultras’ are terrified that their triumph in taking us out of Europe will be snatched away.
“But if that is their fear, why do they not defend their position with logic and passion, with thoughtful, cogent argument, instead of low-grade personal abuse that has been their standard response so far?”
Sir John called for an end to “fake facts and bogus promises” and for others to follow the PM in negotiations with “skill, persuasion and diplomacy” in order to “heal the wounds… of one of the most damaging [debates] in the history of British politics”.
Farron likens PM’s politics to Trump’s and Putin’s}