The party's Corbyn faction and centre-left wing are engaged in an all-out war over what the future of the party should be.
General selection: The best pictures from the UK's parliamentary poll
Boris Johnson said he thought people should serve full prison sentences.
The left-wing Labour party, in contrast to the Conservatives, is de-emphasising Brexit and concentrating instead on domestic issues in its newly-released manifesto.
Watson had long been a thorn in the side for the Labour left.
UK Prime Minister Boris Johnson mocked Labour leader Jeremy Corbyn on Wednesday, comparing his refusal to hold elections thus far to the precepts of late Cuban dictator Fidel Castro.