Either Boris Johnson or Jeremy Hunt will become the new Conservative leader on Tuesday as the result of the contest to succeed Theresa May is announced.
The outcome of the ballot of about 160,000 Tory members will be revealed at just before midday in London.
The victor will officially become prime minister on Wednesday.
Mr Johnson, a former mayor of London, is seen as the clear favourite although a number of senior figures have said they will not serve under him.
Mrs May, who is standing down after a revolt by Conservative MPs over her Brexit policy, will chair her last cabinet meeting on Tuesday morning.
She will officially tend her resignation to the Queen on Wednesday afternoon after taking part in her final Prime Minister’s Questions.