A new referendum on the UK's relationship with the EU is "an option for the future" but "not an option for today", Jeremy Corbyn has said.
The Labour leader confirmed on Sky News that his party would vote against the draft withdrawal agreement.
Mr Corbyn said Labour "couldn't stop" Brexit because it does not have enough seats in Parliament.
Prime Minister Theresa May has said that ousting her would not make it easier to deliver Brexit.
Asked about calls for a further referendum as demanded by some of his MPs, Mr Corbyn said: "If there was a referendum tomorrow what's it going to be on, what's the question going to be?"
If such a referendum were called, Mr Corbyn, who voted Remain in 2016, said: "I don't know how I am going to vote - what the options would be at that time."
Speaking on the Sophy Ridge on Sunday programme, the Labour leader said Mrs May's draft deal with the EU was a "one-way agreement" where the EU "calls all the shots".
It also failed to guarantee environmental protections or workers' rights, he said.
"We'll vote against this deal because it doesn't meet our tests. We don't believe it serves the interest of this country, therefore the government have to go back to the EU and renegotiate rapidly."
Mr Corbyn said Labour would focus on negotiating a permanent customs arrangement with the EU, otherwise the UK would "lose on jobs, lose on investment and we lose on future economic development".
But he admitted he had not read all 585 pages of the draft withdrawal agreement, saying: "I've read a lot of it."
On the same programme, Mrs May said further negotiations with the EU were taking place but that MPs should ensure "we deliver what people in this country voted on".
She warned rivals thinking of replacing her as Conservative leader: "It is not going to make the negotiations any easier and it won't change the parliamentary arithmetic."