Theresa May is to make a surprise statement in Downing Street at 11:15 BST after her Cabinet meets.
BBC political editor Laura Kuenssberg says she has been told by one source the PM will announce a snap general election on 8 June.
Downing Street has always denied Mrs May will call a vote before the next scheduled poll in 2020.
Number 10 has not said what Mrs May will say but statements in Downing Street are usually reserved for big announcements.
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.