Jeremy Corbyn says he is seeking a “constructive alternative” to the PM’s deal, in order to prevent a no-deal Brexit.
The Labour leader was speaking after meeting the EU’s chief Brexit negotiator, Michel Barnier, for talks in Brussels.
It comes ahead of an EU summit where Theresa May will ask EU leaders to postpone Brexit for three months.
Mr Corbyn said he did not believe the PM’s deal “is a way forward”.
“We are therefore looking at alternatives, and building a majority in Parliament that can agree on a future constructive economic relationship with the European Union,” he told reporters after the meeting.
Mr Corbyn was joined by shadow Brexit secretary Sir Keir Starmer for the talks, which also included European Commission Secretary-General Martin Selmayr.