Boris Johnson is under fire for failing to get the approval of the watchdog on business appointments before starting a weekly newspaper column.
The Tory MP wrote his first column for the Daily Telegraph on Monday, a week after he quit as foreign secretary.
Ex-ministers must refer new employment to the Advisory Committee on Business Appointments before accepting it.
The watchdog said it had not heard from him, leading one MP to claim Mr Johnson had broken the ministerial code.
Under existing guidelines, ex-cabinet ministers normally have to wait three months from their last day in office before taking up any outside employment.
Ex-ministers are expected to observe a two-year ban on lobbying government on behalf of any new employer or clients.
The watchdog said it had not had any contact with Mr Johnson. "Ministers are written to when they leave office to remind them of their responsibility to make an application to us," it said.
Mr Johnson, who was Brussels correspondent for the Daily Telegraph before he entered politics, wrote a weekly column for the paper for many years but stopped when he became foreign secretary in July 2016.
Lib Dem MP Layla Moran said its resumption so soon after Mr Johnson quit office left both him and the watchdog with questions to answer.
"The worst Boris faces for this disgraceful behaviour is just a slap on the wrist," she said. "That is disgraceful. Boris has broken the rules yet again, for once he should pay the price for his actions.
"If the watchdog doesn't bark, is it time to put it down?"
The watchdog, which published its annual report on Tuesday, does not have the power to block appointments or take action against politicians who do not observe its rules - leading to claims it is powerless to stop a "revolving door" between government and business.
Last year, however, it criticised former chancellor George Osborne for accepting the post of editor of the Evening Standard before getting its advice.