Dominic Cummings breached government rules by failing to seek permission to launch a blog after leaving Whitehall, a watchdog has suggested.

Lord Pickles, the chairman of the advisory committee on business appointments (Acoba), said Boris Johnson’s former chief adviser failed to notify the panel of the paid-for blog he launched on the Substack website.

Acoba is supposed to vet any appointment taken by ministers or senior officials within two years of leaving office for possible conflicts of interest.

Mr Cummings has used the blog to launch attacks on Mr Johnson and his handling of the Covid pandemic.

He also used it to release WhatsApp messages sent to him by the Prime Minister during his time in Downing Street, in which he appears to threaten to sack Matt Hancock and replace him with Michael Gove and call the former Health Secretary "f—— hopeless".

The blog’s launch followed Mr Cummings’s acrimonious departure from Number 10 in November after an internal power struggle.

Some of Mr Cummings’s posts are free to access, but "more recondite stuff on the media, Westminster, ‘inside No10’, how did we get Brexit done in 2019, the 2019 election etc" is subscriber-only, he said. The blog costs £10 per month or £100 per year.

In a letter to Mr Gove, the Cabinet Office minister, Lord Pickles said it appeared Mr Cummings was offering "various services for payment" through the blog, for which he was also receiving subscriptions. He said it was up to the Government to decide what action to take to enforce its own rules.

"Mr Cummings has failed to seek the committee’s advice on this commercial undertaking, nor has the committee received the courtesy of a reply to our letter requesting an explanation," Lord Pickles wrote.

"Failure to seek and await advice before taking up work is a breach of the Government’s rules. The committee will not advise retrospectively in these circumstances. It is now a matter for the Government to decide what appropriate action to take."

Mr Cummings has said proceeds from the blog will be used to "help [him] provide free help to those campaigning for an immediate, urgent and open Parliamentary inquiry into what really happened with Covid".