The vote had been dramatically postponed last month when Mrs May realised that it would not get through the Commons.
Mrs May has already survived an attempt from hard Brexiteers in her own Tory Party to express no confidence in her.
Mrs May is likely to survive such a vote given Tory and DUP reluctance to do anything that would trigger a General Election that could see them lose power. But further failure to make progress on Brexit will damage her irreparably.
Leader of the Opposition Jeremy Corbyn has called a motion of no confidence which will now be heard tomorrow.
Mrs May lost the crucial vote by 432 votes to 202.
The failure of Mrs May to convince MPs to support her deal casts further doubt on the Brexit process and her own political future now hangs in the balance.
She now has until Monday to present a Brexit Plan B to parliament.