The Conservatives are due to announce who has been elected as their new leader, with David Cameron expected to have beaten David Davis to the post.
The candidates have fought a long and tiring campaign
The postal ballot of 253,600 Tory members nationwide closed on Monday. Counting of the votes started at 0730 GMT, with the result due at 1500 GMT.
Mr Davis, 56, started the campaign as favourite but was overtaken by Mr Cameron, 39, during voting by Tory MPs.
Supporters of Mr Davis insist a victory for their man remains a possibility.
Mr Cameron's supporters have warned the result could be tighter than polls have suggested.
Leaving his west London home on his bike on Tuesday morning, Mr Cameron said he was "never confident until they open those ballot boxes".
Michael Howard announced he would stand down on 6 May, a day after election defeat
In the votes of MPs, Ken Clarke was first to be knocked out, then Liam Fox
David Davis topped the first poll of MPs but was overtaken by David Cameron in the final round
The final decision rests with the 253,600 Tory members
Mr Davis said he was confident. Asked about predictions of a win for his rival, he replied: "You shouldn't count the chickens before they hatch."
The Electoral Reform Society is counting the ballot papers and the result is due to be declared at the Royal Academy in London on Tuesday.
Supporters of Mr Davis say their man must be given a "major role" in the shadow cabinet if he is defeated.
Weekend newspaper reports suggested Mr Cameron was preparing to demote Mr Davis from his current job as shadow home secretary if he won.
But Mr Cameron's campaign manager, shadow chancellor George Osborne, said there were no plans to "humiliate" Mr Davis.
Outgoing leader Michael Howard has urged his party to keep united.
Former leader Iain Duncan Smith, who was ousted by MPs, said the key issue was whether the party was "prepared to give the leader the chance to lead for a considerable period of time and settle in behind him, rather than being there constantly on the sidelines screaming at him".
Several of Mr Cameron's close allies are under-40 but Mr Osborne said the new shadow cabinet would combine youth and experience.
He warned it would be a "long hard slog" to beat Labour - and those expecting an immediate change in the political landscape would be disappointed.
Party members' postal ballots were sent out a month ago. In the weeks since, the candidates have appeared in TV debates and a nationwide series of hustings.
The new leader will face Tony Blair at prime minister's questions on Wednesday.
But most commentators expect Mr Cameron to fight out the next election against Gordon Brown.
Mr Brown dismissed the threat that a young opponent would make him look old.
"I think that's a bit unfair, as the father of a two-year-old I feel pretty young," he told BBC Radio 4's Today programme.
He accused the Tories of "rebranding" old policies like making cuts to public spending.
Mr Cameron, the shadow education secretary, topped a poll of Tory MPs on 20 October.
He took 90 of a possible 198 votes, while Mr Davis, the shadow home secretary, was second on 57.
Rival Liam Fox was eliminated at that stage of the leadership election after gaining 51 votes.
The other contender, Ken Clarke, had been knocked out in an earlier round of voting, which was won by Mr Davis.
The two previous Conservative leaders, William Hague and Mr Duncan Smith, have both backed Mr Cameron for the job.
Outgoing leader Michael Howard, who has held the role since 2003, has refused to name his preferred choice, although Mr Cameron, who worked for him as a special adviser, is believed to be his favourite.
Mr Howard announced he would leave the job shortly after the Conservatives' third general election loss in a row in May.