Theresa May’s Brexit deal is a "miserable failure of negotiation" which will put jobs at risk, according to Jeremy Corbyn.
The Labour leader said his party will oppose the package in Parliament and work with MPs from other parties to block a no-deal Brexit.
His comments came as former prime minister Tony Blair stepped up calls for a second referendum and suggested Labour was shifting towards that position.
On Sunday, EU leaders gave their seal of approval to the deal at a summit in Brussels.
Mr Corbyn described the deal as "bad for the country".
He said: "It is the result of a miserable failure of negotiation that leaves us with the worst of all worlds.
"It gives us less say over our future, and puts jobs and living standards at risk."
Shadow justice secretary Richard Burgon said: "Theresa May needs to is wake up and smell the coffee, ditch this deal and come back with something better."
Labour’s policy is to reject the deal and call for a general election.
If there is not an election, Labour is keeping all options open including the possibility of another referendum on Brexit, although it has not specified what question would be asked.
Mr Burgon told the Andrew Marr Show that another referendum may not even take place until after the UK has left the EU.
He said: "We don’t know when such a referendum, if it takes place, would take place.
"Would it take place before we left the EU? Would it take place after we left the EU?
Speculation about Labour’s position on another referendum has been stoked by a Sunday Times report that shadow chancellor John McDonnell held talks with Tom Baldwin and Alastair Campbell, the former spin doctors to Ed Miliband and Mr Blair, who are now campaigning for a so-called People’s Vote.
Mr Blair told Marr "I think it's moving that way", adding: "I would be really surprised if the Labour Party doesn't end up in the position of supporting another vote, because there’s no other proposition that can get through Parliament."
He suggested another referendum – with the choice between remaining in the EU and a "proper Brexit" of the kind sought by Leave campaigners such as Boris Johnson – would settle the issue.
"Both the Remain campaign and the Leave campaign should jointly agree that this vote is final, doesn’t matter how marginal it is, it's final," he said.
"Once this is resolved then that's it for a generation."
He added that he thought Mrs May was "a decent person, frankly surrounded by a lot of pretty unreasonable ones" but "the deal's a dodo".