An internal inquiry has been launched into how messages showing conversations between the prime minister and billionaire James Dyson were leaked to the press.
However, Downing Street sources cited by The Sun, The Times and The Telegraph have alleged that it is Mr Cummings, who quit No 10 last year following a behind-the-scenes conflict between him and the PM, is behind the revelations.
“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 the prime minister was “saddened” and Mr Cummings was “bitter” after his exit from No 10.
Downing Street’s briefing war against Mr Cummings comes as anger continues to grow around the influence of lobbyists and special interests on UK politics.
Sir James’s texts to the PM saw Mr Johnson assure he would “fix” a tax issue faced by the businessman as his Dyson firm worked to develop ventilators at the height of the pandemic.
In a separate incident, Mr Johnson was sent a text message by Saudi Crown Prince Mohammed bin Salman as a bid to buy Newcastle United ran into difficulties last June.
A No 10 source told The Sun that Mr Johnson “fears Dom was responsible for the text message leaks about James Dyson and Mohammed bin Salman”.
And The Telegraph said it is understood Mr Cummings would have had legitimate access to the messages while he worked at No 10.
“If you join the dots, it looks like it’s coming from Dom,” a source told the newspaper.
Mr Cummings has not responded to the accusations.
Downing Street has said it will publish correspondence between Mr Johnson and Sir James “shortly”, after the prime minister told the Commons he was “happy to share all the details” of the exchanges.
The prime minister’s official spokesperson said: “The prime minister said in the House he’s happy to share all the details with the House, as he shared them with his officials.
“That’s what we’re working on, we’re pulling together that information, it will be published shortly.”
Meanwhile, the spokesman did not deny reports that the cabinet secretary, Simon Case, advised Mr Johnson to change his phone number over concerns about the ease with which lobbyists and business leaders were able to contact him.
The spokesperson told Westminster reporters: “We don’t get into details of the advice provided between a cabinet secretary and a prime minister, and so I’m not going to do that in this instance.”
Additional reporting by PA