Theresa May is to make an unscheduled statement in Downing Street at 11:15 BST after her Cabinet meets.
In an unusual move, Number 10 did not announce what subject she would address but statements by the PM in the street are usually reserved for big announcements.
There is speculation that Mrs May could call a snap general election.
Downing Street has always denied she will call a vote before the next scheduled poll in 2020.
BBC assistant political editor Norman Smith says broadcasters usually know what is about to be announced, but on this occasion they do not.
“Prime ministers only make Downing Street statements when it’s something significant,” he said.
These statements can cover resignations and elections, but he added: “We do not know.”
The Fixed-Term Parliaments Act sets the general election date as the first Thursday in May every five years, meaning 2020 is the next expected contest.
But Mrs May could call an early election if two-thirds of MPs in the Commons vote for it and Jeremy Corbyn has previously indicated Labour would support such a move.
Speculation rife over PM statement at Number 10}