Dominic Cummings has been accused of breaking government rules with his pay-monthly blog.
The former Downing Street advisor created a profile on Substack, the blogging platform, last month in a bid to attack Boris Johnson, his former colleagues and their handling of the Coronavirus pandemic.
Lord Pickles, chairman of the Advisory Committee on Business Appointments (Acoba), slammed Mr Cummings for failing to notify the panel of the blog.
Official rules state former senior advisors and ministers must seek advice from ACOBA before taking up new roles for two years after leaving office.
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Mr Cummings set up his Substack blog with subscriptions said to cost either £10 a month or £100 a year.
He made his first post on the blog free - when sharing Johnson's disparaging texts on former Health Secretary Matt Hancock.
Johnson's former Chief of Staff said the Prime Minister considered “taking Hancock off and putting [Michael] Gove on”, and in a text message told Cummings that Hancock is “totally f***ing hopeless.”
Lord Pickles said it appeared Mr Cummings was offering "various services for payment" through the blog, for which he was also receiving subscription payments.
In a letter to Cabinet Office minister Michael Gove he said: "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."
"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' blog followed his acrimonious departure from No 10 last November after an internal power struggle with the Prime Minister's then fiancee - now his wife - Carrie Symonds.
His exit came after 16 months rising to power in Downing Street and perhaps becoming the most high-profile adviser of recent times in UK politics.
