Jeremy Corbyn called for the European Union to be "defeated" in explosive comments unearthed by talkRADIO.
Mr Corbyn told a left wing rally in 2010: "They, the world's bankers, International Monetary Fund, European Union, they are utterly united in what they want. Utterly united in deflation, suppressing the economy, and creating unemployment. Utterly united in that.
"We need to be equally united, not just across every union in this country and every community in this country and every social demand in this country, but all across Europe and internationally to show that the voice of those campaigning for peace, justice and socialism.
"We will not be silenced by these people. We will win through. We will defeat them and we will win that decency that we want in this world."
Mr Corbyn's remarks were made at the Durham Miners Gala, a trade union gathering that the Labour leader attends annually.
Mr Corbyn's speech will infuriate many of his own MPs and Labour members who back remaining in the EU.
A petition calling on Mr Corbyn to back a second referendum today reached 50,000 signatures.
Labour MP Angela Smith was turned away from Labour HQ when she attempted to hand in the petition this afternoon.
'That's what ordinary members believe in'
Owen Smith, the Labour MP who challenged Mr Corbyn in 2016, told talkRADIO: "My party must realise the nightmare of a Tory Brexit is growing by the day. We need to force the government to take no deal off the table and support the public being given the final say.
"That’s what ordinary members believe in, and what the majority of Labour voters around the country want. Leaving the EU isn’t going to help neglected communities across the country."
Mr Corbyn's precise views on the European Union have long been the subject of speculation.
Tom Brake, the Liberal Democrat Brexit spokesperson described the speech as "the true Jeremy".
"This is the true Jeremy, spouting absurd conspiracy theories about the EU," he said.
"It is time he abandoned his prejudices and accepted the EU has been a major guarantor of workers' rights and environmental standards.
"If he wants to secure British jobs stopping Brexit through a People's Vote is the best way to do it."
A Labour Party spokesperson said: "Jeremy was speaking out against the austerity that the IMF, the EU and the world’s bankers were forcing on the people of Europe at the time and calling for people to work across borders to defeat austerity and build an economy that works for the many not the few.
"He is proud of his longstanding record of campaigning for internationalism, social justice and for an investment-led economy."