The Labour leader met his reshuffled top team at Westminster on Monday for the first time following the party's damaging losses in last week's elections.
It followed intense criticism over the weekend after he stripped deputy leader Angela Rayner of her roles of party chairwoman and campaign co-ordinator.
Amid furious accusations he was trying to make her a scapegoat, it was announced late on Sunday she would be given a new role shadowing the Cabinet Office minister Michael Gove.
She was also made shadow first secretary of state, effectively making her the shadow deputy prime minister.
Labour sources said Sir Keir made clear during the meeting on Monday that he was not trying to shift the blame for the party's dismal election showing.
He was said to have told them there was no escaping the scale of the defeats, which said "something profound about the size of the journey we have to go".
He said: "To be clear, I take responsibility. Nobody else.
"I lead the Labour Party and it is entirely on me."
Sir Keir said he had given Ms Rayner "a big new role, taking the fight to the Tories".
In other changes shadow chancellor Anneliese Dodds was demoted to party chairwoman, to be replaced by Rachel Reeves who was previously shadowing Mr Gove.
Veteran chief whip Nick Brown also stood down to be replaced by Alan Campbell.