MPs voted by 432 votes to 202 to reject the deal, which sets out the terms of Britain's exit from the EU on 29 March.
Labour leader Jeremy Corbyn has now tabled a vote of no confidence in the government, which could trigger a general election.
The confidence vote is expected to be held at about 1900 GMT on Wednesday.
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Mr Corbyn said it would allow the House of Commons to "give its verdict on the sheer incompetence of this government".
But DUP leader Arlene Foster said her party, which keeps Mrs May in power, would be supporting her in Wednesday's confidence vote.
She told the BBC MPs had "acted in the best interests of the entire United Kingdom" by voting down the deal.
But she added: "We will give the government the space to set out a plan to secure a better deal."
Some 118 Conservative MPs voted with the opposition parties against Mrs May's deal.
Only three Labour MPs supported the prime minister's deal: Ian Austin (Dudley North), Kevin Barron (Rother Valley) and John Mann (Bassetlaw).