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.
He is also expected to meet seven European leaders at the two-day summit, which begins later.
What do people actually make of this? While Ive said many times he should be trying to make a difference - it doesnt sit well with me that he is off round europe trying to negotiate separately from the Prime Minister.
And this isnt a Corbyn bashing thing, before certain posters get their knives out, this is more - should the opposition actually be undermining the countries leader like that...? thing.