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.
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, is chairing her last cabinet meeting.
She will officially tender her resignation to the Queen on Wednesday afternoon after taking part in her final Prime Minister’s Questions.
Her successor will take office shortly afterwards, following an audience at Buckingham Palace.
The embedded expectation in Westminster is that the name will be Boris Johnson – unless the Tory party has been collectively deceiving itself in the past few weeks.
If it proves so, the triumph will be extraordinary. Not because of a journey Mr Johnson has been on in the last few weeks – the controversial former foreign secretary and London mayor started out as the frontrunner.
But because again and again, over many years, his own political accidents and behaviour would have ruled other politicians out.
Mr Johnson’s supporters would say he has found himself in some serious scrapes.
His detractors would say he has blundered his way through a high-profile career causing offence and putting his own interests ahead of the country’s.
It wasn’t so long ago that the same received wisdom in Westminster that said he could never make it, said that he had blown too many chances – his long held public ambition would never be achieved.
But it is likely his status as Brexit’s cheerleader-in-chief will see him into the job he has craved.
Conservative members have been voting by post for the past two-and-a-half weeks. It is the first time they will have selected a serving prime minister.
Since he made the final two candidates last month, Mr Johnson – who led the Leave campaign in the 2016 Brexit referendum – has been regarded as the clear frontrunner.
Conservative MP Sir Michael Fallon said, Mr Johnson would “improve” the Brexit deal with the EU in a way that would satisfy Parliament.
“One of the great attractions of Boris taking over our party is that he is optimistic and ambitious.