An internal inquiry has been launched into how messages between the Prime Minister and billionaire entrepreneur Sir James Dyson were leaked to journalists.
The leaked texts show Mr Johnson promising the businessman he would "fix" a tax issue for Dyson staff working to develop ventilators at the height of the coronavirus crisis last year.
Now reports said Downing Street sources are pointing the finger at Mr Cummings, who quit as the Prime Minister's senior adviser last year months after his derided rule-breaking trip to Durham at the height of lockdown.
Following a reported internal power struggle with the Prime Minister's fiancee, Carrie Symonds, Mr Cummings was ousted from No 10.
The Times, Daily Telegraph and Sun all reported comments from an insider naming Mr Cummings.
"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.
This leak is not the first time the Prime Minister's messages have been made public.
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".
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.
No 10 had initially said there would not be a probe into how the exchange with Sir James was made public, but a change of course was announced on Thursday as it said an internal inquiry will be led by the Cabinet Office.
The spokesman confirmed the inquiry will examine the source of leaks of Mr Johnson's private communication "as related to this issue of Dyson".