“I think the Prime Minister has been asleep at the wheel, he has been slow, the comms, the communications has been terrible,” Mr Starmer told Good Morning Britain.
“It began to break down frankly when Dominic Cummings put forward a ridiculous defence of what he had done in the north east.”
He added: “They’re losing control of the messaging.”
Mr Starmer said he is “sweating blood” to rebuild public trust in his party, despite a recent poll suggesting that public opinion favoured his leadership over that of the current Prime Minister Boris Johnson.
Asked if he thought he would make a better prime minister than Mr Johnson on Sky News, Sir Keir said: “I focus on the fact that last December the Labour Party lost very badly in a general election, and that’s our starting point.
“I’ve got a mountain to climb to get our party from where it is to where it needs to be to win the next general election, and I’m sweating blood on that.
“I’m not going to be deflected from the enormity of the task that I face. I’ve got to rebuild trust in the Labour Party, trust in a force for good and a force for change.
“I’m determined to do that, but I’ve got a long long way to go.”