The vote is expected to take place today.
Speaking outside 10 Downing Street, Mrs. May said, “She will contest that vote with everything she has got”.
She said a new Prime Minister would have to scrap or extend Article 50, the mechanism taking Britain out of the EU on 29th March, “delaying or even stopping Brexit”.
A result is expected fairly quickly after the voting finishes.
If Mrs. May does not win the vote there would then be a conservative leadership contest in which she may not stand.