Boris Johnson is facing questions over whether he followed rules on paid employment after leaving No 10 after receiving $150,000 (£135,000) for a speech to a group of US insurance brokers. The former prime minister gave a speech to the Council of Insurance Agents and Brokers in Colorado Springs this week, only just over a month after leaving Downing Street. Johnson did not seek approval from the Advisory Committee on Business Appointments (Acoba) before giving the speech. Sources close to the former prime minister suggested there was no need because it was a one-off. Former ministers need to apply for permission to take up outside employment only if they have signed an ongoing contract with a business, such as a speakers’ agency. Cabinet ministers are also expected to wait three months before taking up employment. Johnson was recently added as an available speaker to the website of an agency called the Premium Speakers Agency but his profile disappeared and those close to him claim he was mistakenly listed. Angela Rayner, Labour’s deputy leader, challenged Johnson to prove that he has stuck to the rules. “Boris Johnson might claim that this was just a one-off but the rules state that ex-ministers … joining agency circuits or scribing newspaper columns must submit an application before accepting them. “The disgraced former prime minister and now part-time MP once again has questions to answer about whether he has followed the rules he was once responsible for. “The anti-corruption watchdog was already toothless, but under the Tories, it’s been muzzled and neutered, leaving an open door to former ministers who want to line their pockets as soon as they leave office. Labour will clean up politics by ensuring proper and enforceable sanctions for rule breaking and banning former ministers from lobbying government for at least five years after they leave office.” The Times reported on Wednesday that Johnson’s Colorado speech was greeted with a standing ovation. He made jokes about it having been expensive for him to have been born in the US – he renounced his US citizenship in 2016 but prior to that had faced hefty tax demands – and gave his assessment of the situation in Ukraine. His appearance involved a 30-minute speech and a 45-minute “fireside chat” but no questions from the audience. Johnson also set up his own office this week, likely to be the vehicle for accepting earnings, registering The Office of Boris Johnson Ltd with Companies House. Johnson previously broke Acoba rules when he failed to declare a column from the Daily Telegraph after leaving office as foreign secretary. However, there are no formal sanctions for a breach of the rules, leading to accusations that Acoba is a toothless watchdog. John Penrose, Johnson’s former anti-corruption tsar, has suggested ministers should have to sign legal deeds agreeing to abide by Acoba’s rules before taking office. He told the Guardian earlier this year: “Acoba isn’t fully legally binding at the moment, and it ought to be. So what [the Boardman review] has suggested is that civil service contracts should make Acoba’s decisions binding and, because ministers aren’t technically employees, the equivalent for them is that they sign a legal deed that says: ‘I will be bound by the decisions of Acoba.’ It’s a nice, simple way of giving Acoba the teeth and claws it needs.”