Shadow Brexit secretary Keir Starmer has said threats of leaving the European Union without a deal are a "political hoax".
Starmer, who has called for a second referendum or people's vote, said MPs were not duty-bound to vote through a "bad deal" in parliament and must instead "take back control".
Writing in the Sunday Times, Starmer said the prime minister was serving MPs a "choice between bad and even worse".
"Well, prime minister, that’s not good enough," he wrote. "There is no duty on MPs to surrender to a bad deal. To do so would be to concede to a political hoax designed to threaten rather than persuade."
The Labour frontbencher said there was "no mandate" for a no-deal Brexit, in which the UK would crash out of the EU and default on trade terms set by the World Trade Organisation.
He said the way out of such an eventuality was to table amendments to May's Brexit deal or a no-confidence vote in Theresa May.
The prime minister is currently facing the prospect that the deal she brings back from the EU will be voted down, including by members of her own party.
Sammy Wilson, the DUP's Brexit spokesman, today wrote a joint article with Tory MP Steve Baker in the Sunday Telegraph in which they branded the current plans to avoid a hard border in Northern Ireland as a "humiliation".
"If the government makes the historic mistake of prioritising placating the EU over establishing an independent and whole UK, then regrettably we must vote against the deal," they wrote.
Starmer said Labour is prepared to vote down the deal. "Labour will stick to its guns. Supporting a bad deal is not in the national interest," he said.
Labour has consistently pushed for a general election to solve the current impasse in negotiations. Shadow foreign secretary Emily Thornberry told the BBC this morning that her party would be in a better position to negotiate with the EU because it did not sign itself up to the "stupid red lines" that formed the basis of the prime minister's negotiating strategy in 2016.
However, she admitted that Labour had not any "serious discussions" with EU officials about how they might negotiate a deal.