The architect of the Leave campaign said he had reluctantly decided to back her deal so we can leave the EU in May.
He wrote on Twitter: It is very painful to vote for this deal.
But I hope we can now work together to remedy its defects, avoid the backstop trap and strive to deliver the Brexit people voted for.
The Prime Minister said she would stand down if she can get her deal passed.
Mr Johnson is among the front-runners to replace her, although he is yet to officially throw his hat into the ring.
His colleague Jacob Rees-Mogg has also been a vocal critic of Mrs Mays but he has yet to say which way he will vote.
He said he would vote in favour if the DUP back it or abstain.
All 10 DUP politicians are set to vote against Mrs May because of the Irish backstop issue, which they say will see Northern Ireland treated differently to the rest of the UK.
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