Johnson's former chief adviser made the comment in a blog post on Tuesday, as Johnson's 52-page dossier of defence over the Partygate allegations was made public.
Johnson is accused of lying to Parliament over gatherings he hosted at 10 Downing Street during lockdown - he claims that he did not intentionally mislead Parliament and has insisted he did not believe he was breaking any rules at the time.
In his evidence today, Johnson took direct aim at Cummings - who has previously accused his former boss of knowingly breaking rules - saying he "lacked credibility" because of prior critical comments.
In response, Cummings outlined in the blog post a gathering he referred to as "the ABBA party", which he claimed was not investigated properly.
He has claimed the Downing Street summer event produced a number of fines for female, junior members of staff, while Johnson escaped censure.
"A number of junior women were told to attend the BYOB event summer 2020 by the PM’s PPS [personal private secretary] who a) was responsible for checking the rules with the covid taskforce and b) told officials it was a ‘work event’," Cummings wrote.
"The PM was NOT fined for attending, even though it was his decision to hold the event, but the junior women, who reasonably thought ‘the covid taskforce has approved this event’*, were fined," he claimed.
"There are many such episodes in this sorry saga," Cummings continued. "The civil service talks a lot about being a great employer, and its ‘values’, but the truth is it repeatedly treats junior people shamefully, cannot keep young talent, and has many people in senior management with no talent or moral courage. They are the people who’ve survived the moral maze of a rotten HR and promotion system, with the ‘values’ and priorities you’d expect."
According to Cummings, Downing Street was treated as a "test area" and, as a result, exempt from some COVID rules at the time. This meant junior members of staff would have no reason to believe the event was unlawful if they had been told by the prime minister's staff to attend.
Johnson will face off against MPs on Wednesday in a moment that could throw his political career into jeopardy.
The evidence, made public on Tuesday, shows Johnson accepted he misled Parliament but claimed he did not do it intentionally.
Johnson admitted he had made “honest but inadvertently misleading statements” that he had later corrected.
“I was not trying to conceal these events because I believed that there was nothing to conceal or cover up,” he said, also lashing out at the cross-party committee for being “absurd, illogical and partisan”.
Cummings has been highly critical of Johnson since leaving his post.
In his evidence published on Tuesday, Johnson said of his former adviser: "It is no secret that Dominic Cummings bears an animus towards me, having publicly stated on multiple occasions that he wanted to do everything that he could to remove me “from power”. He cannot be treated as a credible witness."