The former party leader criticised his successor on Twitter after the loss of the Hartlepool by-election and scores of council seats.
Sir Keir has sought to shift Labour more to the centre-ground since taking over as leader last April.
Attacking the decision, Mr Corbyn accused Sir Keir for a “loss of hope” and he urged the leadership to offer a more radical policy agenda.
He said: “Tory gains are bad news for jobs, the environment & public services for the many not the few.
“With millions not voting, these results show a loss of hope.
“We must offer a bolder vision to transform people’s lives and give them the confidence to strive for a more equal world.”
More to follow…