Earlier this week, public broadcaster the BBC reported on leaked messages between Johnson and Sir James Dyson. Several British media outlets have suggested Cummings was the source of the leaks.
"I was not directly or indirectly a/the source for the BBC/Kuenssberg story on the PM/Dyson texts," wrote Cummings in his blog post. "I am happy to meet with the Cabinet Secretary and for him to search my phone for Dyson messages. If the PM did send them to me, as he is claiming, then he will be able to show the Cabinet Secretary on his own phone when they were sent to me. It will therefore be easy to establish at least if I was ever sent these messages."
Cummings, a divisive figure who has often been painted as the mastermind behind the Brexit campaign and Johnson's rise to the premiership, resigned from his position as chief adviser to the prime minister last November amid tumult within the UK's Conservative government.
Downing Street has launched an inquiry into the source of the leaked text messages and other leaks.
The text exchange between the British prime minister and Sir James Dyson occurred last March during the UK's first wave of coronavirus, after Johnson's government asked companies to help manufacture thousands of ventilators for hospitalized Covid-19 patients.
In a text message, Johnson said he would "fix" concerns Dyson had about a change in tax status for his Singapore-based staff if they came to the UK to make ventilators, according to the BBC report.
When asked on Friday if he blamed Cummings for the leak, Prime Minister Boris Johnson said, "I think people aren't much interested in who's briefing what, as the substance of the issue at hand. The issue is really the question of the ventilators, if you remember, that James Dyson was offering to make. And let's be absolutely clear that I think he was right to talk to him, I think he was right to talk any British manufacturer."
On Friday, a 10 Downing Street spokesperson said in a statement, "This government is entirely focused on fighting coronavirus, delivering vaccines and building back better."
Cummings' blog post also alleged that the prime minister was thinking about canceling an inquiry concerning the leaked announcement of a further UK lockdown last October.
Cummings claims the prime minister wanted to cancel the leak inquiry due to concerns that it would implicate someone who was friends with Boris Johnson's fiancée, writing in the blog post that he told Johnson "this was 'mad' and totally unethical."
A Downing Street spokesperson said in a statement, "The PM has never interfered in a government leak inquiry."
Cummings added, "I have made the offer to hand over some private text messages, even though I am under no legal obligation to do so, because of the seriousness of the claims being made officially by No10 today, particularly the covid leak that caused serious harm to millions. This does not mean that I will answer every allegation made by No10."