Boris Johnson has committed a “clear breach” of the ministerial code by not clearing a new role in writing a column for the Daily Mail with the parliamentary authorities.
The committee that vets ex-ministers appointments says he informed them only half an hour before the news emerged.
The first weekly column by the former prime minister appeared online late on Friday afternoon.
The paper was one of Mr. Johnson’s staunchest supporters when he was PM.
Mr. Johnson resigned as an MP on Monday but is still required to seek advice from the Advisory Committee on Business Appointments (Acoba) on new jobs for two years after leaving ministerial office.
He stepped down as prime minister last September.
In a statement, Acoba said: “The ministerial code states that ministers must ensure that no new appointments are announced, or taken up before the committee has been able to provide its advice.
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“An application received 30 mins before an appointment is announced is a clear breach.
“We have written to Mr. Johnson for an explanation and will publish correspondence in due course, in line with our policy of transparency.”
A spokesman for the former prime minister said: “Boris Johnson is in touch with Acoba and the normal process is being followed.”
Describing Acoba as “toothless”, deputy Labour leader Angela Rayner said Mr. Johnson was “once again breaking the rules and taking advantage of a broken system for his benefit”.
Acoba, chaired by Conservative peer Lord Pickles, exists to ensure there is “no cause for any suspicion of impropriety” when a former minister or senior official takes up a new job.
It is currently looking into the case of former Partygate investigator Sue Gray, who quit the civil service in March, having been offered a post as Labour leader Sir Keir Starmer’s chief of staff. Both Ms. Gray and Labour have said they will abide by Acoba’s recommendations.
The committee has no powers to enforce its recommendations or to punish MPs – or former MPs – who have broken the rules, but it can issue public rebukes.
Earlier, an Acoba spokeswoman told the BBC “newspaper columns are not considered significantly problematic”, but Mr Johnson was still meant to seek its advice.