His Labour party had a landslide election victory in 2024.
He'd been under growing pressure to step aside since his main leadership rival Andy Burnham secured a seat in parliament.
Britain is now on the verge of getting its seventh prime minister in a decade.
Our UK political correspondent Rob Watson tells us why things went wrong for Keir Starmer so quickly.
The root cause of all this political instability is that damn Brexit! Fie on Farage! I'm not at all sure that Andy Burnham will do much better than Starmer, either. Marginally, perhaps. But until we get back into the European Union, this instability is likely to be the order of the day. — © Mark Alexander
