Former Conservative leader William Hague is backing David Cameron to become the next party leader.
Mr Hague says he has already filled in his ballot paper
Mr Hague has already cast his vote and publicly declared his support in a News of the World column.
He said the 39-year-old was the best candidate to modernise the Tory party, inspire voters and challenge Labour.
Defeated leadership challenger Liam Fox has also backed Mr Cameron over David Davis, although he told the Sunday Times it was a "close call".
Mr Hague said Mr Cameron had "a certain quality" that was impossible to define, "but you know it when you see it".
He had previously refused to say who he believed should replace Michael Howard and said he had thought "long and hard" before casting his vote.
In his column on Sunday, Mr Hague said Mr Davis was "forceful, energetic and determined", but criticised him for committing himself to tax cuts whatever the state of the economy.
'Pass the torch'
Shadow foreign secretary Dr Fox was the last candidate to be eliminated when MPs decided on the final two to go forward to grassroots members.
He told the Sunday Times: "I have decided to back David Cameron because I think the best approach for us to win is one of greater collective decision making and leadership which accepts that it is important to return to a concept of first among equals.
"There is an opportunity for us to pass the torch to the next generation of Conservative leadership which can show that it is not about personal ambition, but a collective wish to see a better agenda for the Conservative Party that will decide a better future for the British people."
Mr Davis and Mr Cameron will face one another in front of an audience of activists on Monday, at the first of a series of appearances around the country.