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.
Mr Corbyn has faced criticism after walking out of a Brexit meeting with the PM on Wednesday because Labour defectors, who are now members of the Independent Group, turned up.
Independent Group spokesman Chuka Umunna described the Labour leader's behaviour as "juvenile" at a time of national crisis.
After the meeting, other opposition party leaders said they were unimpressed with what they heard from the prime minister.
Mr Corbyn said there had been "a confusion" over the meeting, and he had held separate discussions with Mrs May later on.
"I'm also arranging to meet the prime minister next week again on a one-to-one basis," he added.
Labour has backed an extension of Brexit talks to find an alternative to the prime minister's deal which will command a majority in the Commons.
On Wednesday, Brexit Secretary Stephen Barclay said Labour's plans were "not credible", and the party was asking for "things that are simply not on offer".