Cummings, Johnson’s former chief adviser, said the email was sent on 26 April last year, just under a month after COVID-19 hospital admissions had peaked in the first wave.
Johnson apparently complained how building the test and trace system “feels like whistling in the dark”, and that there was “no idea how to get new cases down to a manageable level”.
Cummings also said Johnson raised a fear that the UK could have the most COVID-19 deaths in Europe, as well as the worst-hit economy.
He is said to have concluded the email with: “We GOTTA turn it round.”
The apparent date of the email was when Johnson was staying at Chequers, the PM's official country residence, while recovering from his own brush with death from COVID.
Here is a screenshot of what Cummings said was Johnson’s email on 26 April…
Cummings’ leak, in his latest Substack post, comes nine days after he published a WhatsApp message screenshot from “Johnson Boris” in which health secretary Matt Hancock is called “totally f***ing hopeless”.
Cummings left Downing Street in November last year and has repeatedly attacked Johnson's handling of the pandemic in the past couple of months.
In the latest blog, he said the PM was responding to an email Cummings had sent to officials in which he complained about the state of the test and trace system and data for COVID testing.
Watch: Five of the most important moments from Dominic Cummings' testimony last month
In the email, Cummings said he hadn't seen a detailed plan for the launch of NHS Test and Trace, which was seen as a key way of controlling infections. It eventually launched on 28 May but there were well-documented questions about its effectiveness.
On testing data, Cummings also said key policy meetings were "happening in a largely data-free way so neither officials nor ministers can really get to grips with the choices, implications..."
Cummings posted the blog, entitled "More evidence on how the PM's & Hancock's negligence killed people", on Friday afternoon after Hancock apologised for breaking social distancing rules. A photo, published in The Sun, showed him kissing a close aide.
Johnson refused to sack Hancock for the rule breach, with Downing Street saying the PM has "accepted" Hancock's apology "and considers the matter closed". Asked if Johnson has “full confidence” in him, a spokesman replied: “Yes.”
Cummings has repeatedly told how Johnson ignored his pleas to sack Hancock for his performance at the beginning of the pandemic.
Watch: Hancock resists calls to resign over reports of affair with close aide