The Conservative Party announced at breakfast time that it had reached the 15 per cent threshold of MPs seeking a vote.
Members will now hold a ballot between 6pm and 8pm tonight, with the votes being counted straight away.
Sir Graham Brady, chairman of the 1922 Committee, issued a statement saying: "The threshold of 15 per cent of the parliamentary party seeking a vote of confidence in the leader of the Conservative Party has been exceeded.
"In accordance with the rules a ballot will be held between 1600 and 1800 today 6th June - details to be confirmed.
"The votes will be counted immediately afterwards. An announcement will be made at a time to be advised.
"Arrangements for the announcement will be released later today."
Sources say the Prime Minister was told yesterday that he would be facing a vote.