Jeremy Corbyn has vowed outright opposition to the Government's welfare reforms, branding the Tories "poverty deniers".
It marks a major shift from July, when acting leader Harriet Harman ordered Labour MPs to abstain on the first reading of Work and Welfare Reform Bill.
Ms Harman and some of her fellow shadow cabinet ministers, including Mr Corbyn's leadership rival Liz Kendall, backed certain measures of the Government's cuts to the welfare bill, such as lowering the benefit cap to £23,000 and cutting child tax credits.
But in his first official speech as Labour leader, Mr Corbyn pledged to "oppose the Welfare Bill in full" and accused the Tories of attempting to impose "social cleansing" on the poorest in society.
He vowed to "remove the idea of the benefit cap altogether", announcing that scrapping it was now Labour party policy.
He received a standing applause as he spoke to trade union delegates at the TUC conference in Brighton - an event that has proved tricky for Labour leaders in recent years.
But Mr Corbyn won loud applause for accusing the Tories for "declaring war on organised Labour" and told trade unions that the Government sees them as "the enemy within".
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"They think they will put me and Labour on the back foot by highlighting our support for trade unionism," he added.
“I am a proud trade unionist. We will fight this Bill all the way, and if it becomes law we will repeal it in 2020."
He also attacked David Cameron and George Osborne for continuing their programme of austerity as he vowed to fight the welfare reforms in their entirety.
“They call us deficit deniers. But then they spend billions cutting taxes for the richest families or for the most profitable businesses.
“What they are is poverty deniers: Ignoring the growing queues at food banks. Ignoring the growing housing crisis. Cutting Tax Credits when child poverty rose by half a million under the last Government to over four million.
“Labour will oppose the Welfare Bill in full. We oppose the benefit cap. We oppose social cleansing.
“We will bring the welfare bill down by controlling rents and boosting wages, not by impoverishing families and socially cleansing our communities.”
