Jeremy Corbyn has used his New Year's message to warn against a Brexit deal that only protects the City.
The Labour leader said 2016 "will be defined in history" by the decision to leave the European Union (EU) and said the party "won't stand by" and allow Theresa May to negotiate a deal it does not agree with.
Making a populist pitch, Mr Corbyn blamed the political system for "letting down the people of this country" and said he will take on the establishment.
Taking stock of the Brexit vote, he said: "People didn't trust politicians and they didn't trust the European Union. I understand that.
"I've spent over 40 years in politics campaigning for a better way of doing things, standing up for people, taking on the establishment and opposing decisions that would make us worse-off."
He promised Labour will not try to block the referendum result but "won't stand by" either.
He said: "Those in charge today have put the jobs market, housing, the NHS and social care in crisis. We can't let them mess this up. It's about everyone's future.
"A Brexit that protects the bankers in the City and continues to give corporate handouts to the biggest companies is not good enough."
Strategists are reportedly trying to relaunch Mr Corbyn as a left-wing populist in a bid to capitalise on the anti-establishment feeling sweeping through politics.
Mr Corbyn used his message to attack the Government over its record on the NHS, social care and homelessness, which has risen sharply in recent years.
He said the political system is "letting down the people of this country" and that "decisions made in Westminster are making people's lives harder".
He added: "Whether that's elderly people not receiving the care at home they deserve, putting huge strain on them and their family, or whether it's the people waiting longer in A&E or on trolleys because our National Health Service and social care system is at breaking point, despite the best efforts of the wonderful and dedicated staff.
"Whether it's the homeless families who are being priced out of a housing market that only works for the few."
He said many Britons are struggling with the twin burdens of insecure housing and insecure work.
"Millions of people can't plan their lives because, whether on temporary or zero-hours contracts, they don't know what job or what hours they'll have from day to day, week to week or month to month", he said.
"For many, pay is so low that it doesn't make ends meet."
Mr Corbyn said Labour was "founded to stand up for people" and created institutions such as the NHS which improve the daily lives of millions of people.