British Prime Minster Keir Starmer is expected to resign and set out a timetable for his departure, the Observer newspaper reports.
However, a government source said Starmer remained focused on getting on with the job of governing.
The threat to Starmer's position, which has been building for months, increased sharply on Friday when his rival Andy Burnham won a seat in parliament that would allow him to launch a formal leadership challenge.
The Observer said Starmer would quit on Monday after reaching the conclusion that his position was no longer tenable after speaking to cabinet minister, advisers, donors and trade union leaders.
Starmer was discussing the matter with his wife Victoria at his Chequers country residence before making a final decision, but that senior Labour figures expected a clear statement on his future as early as Monday, the report said.
However, a government source said Starmer remained focused on his job and pointed to previous statements he had made to that effect.
The British leader said on Friday he would fight any challenge to his position and urged Labour not to tear itself apart with infighting.
More than 100 MPs in his party - roughly a quarter of all Labour representatives in the House of Commons - have publicly said they want Starmer to quit or set out a timetable for his departure.
