Jeremy Corbyn is urging socialist and progressive parties across Europe to rally to counter the rise of the "populist right".
In a speech to the Party of European Socialists in Prague on Saturday, the Labour leader will warn that unless progressives break with a "failed economic and political establishment" the far right will fill the gap.
His intervention comes as the populist right has been emboldened by the vote for Brexit and the success of Donald Trump in the US, with the Freedom Party challenging for the presidency in Austria and Marine Le Pen's National Front hoping to do well in France.
In his address, Mr Corbyn will say that while the populist right had identified many of the "right problems" at a time of growing insecurity and declining living standards, the solutions they offered were the "toxic dead-ends" of the past.
"They are political parasites feeding on people's concerns and worsening conditions, blaming the most vulnerable for society's ills instead of offering a way for taking back real control of our lives from the elites who serve their own interests," he will say.
"But unless progressive parties and movements break with a failed economic and political establishment, it is the siren voices of the populist far right who will fill that gap."
The Labour leader will point to the economic conditions which had proved such fertile ground for the populist right to exploit.
"We know the gap between rich and poor is widening; we know living standards are stagnating or falling and insecurity is growing; we know that many people rightly feel left behind by the forces unleashed by globalisation, powerless in the face of deregulated corporate power," he will say.
On Brexit, he will repeat his call for the Government to guarantee the rights of EU citizens living in Britain before it opens formal negotiations for withdrawal under Article 50.
He will announce that he is inviting leaders of socialist and progressive parties to a conference in London to discuss the best outcomes for both Britain and the remaining EU.
"Labour is pushing for Brexit negotiations to be carried out in a transparent manner, in a spirit that aims to find a deal that works for all people across our continent," he will say.
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