He insisted that he would “not comment on speculation”, despite stories in the Times, The Daily Telegraph and The Sun quoting Downing Street sources claiming Mr Cummings leaked the texts.
Asked if the PM thought Mr Cummings was leaking content from when he was in Government, the PM’s spokesman said: “We’ve instructed the Cabinet Office to look into the Dyson leak…obviously I’m not going to comment on speculation and of course it is for the Cabinet Office to take that work forward. I’m not going to pre-empt it.”
He was asked if he recognised the briefings that had been given to the papers last night but repeated that he would “not comment on speculation”.
Asked if he was denying that the briefings had come from No10, he replied: “I’m simply saying that I’m not commenting on speculation. That’s all I’m saying.”
He also avoided a question on when the PM had last spoken to Mr Cummings and said he would not get into details of any private conversations Mr Johnson may have had.
Reports overnight said Downing Street sources were pointing the finger at Mr Cummings, who quit as the Prime Minister’s senior adviser last year following a behind the scenes power struggle at No10.
“Dominic is engaged in systematic leaking,” a source told The Times.
“We are disappointed about that. We are concerned about messages from private WhatsApp groups which have very limited circulation.”
The source suggested Mr Johnson was “saddened” and Mr Cummings was “bitter” after his exit from No10.
Former Vote Leave mastermind Mr Cummings has not responded to the claims.