Her appointment came after she had courted controversy in her previous role as the commissioner for victims and survivors in Northern Ireland. In 2013, she gave an interview to the Belfast News Letter in which she refused to be drawn on whether she believed the IRA and UVF were terrorists. She went on to become commissioner for the Independent Police Complaints Commission, before beginning her part-time tenure in Parliament. 

Ms Stone quickly established herself by resuming an investigation into Keith Vaz, the former Labour MP first started by her predecessor, Kathryn Hudson, in October 2016, which followed revelations that the former chairman of the home affairs select committee had partied with Romanian male escorts. 

There was widespread disbelief, however, when the initial investigation was suspended “on medical grounds” in December 2017. Critics pointed out that the then for Leicester East had been well enough to travel to India and Saudi Arabia as well as being spotted at several “ribbon cutting events”. 

It was not until nearly two years later that Mr Vaz was suspended for six months, after the long-running probe finally concluded there was “compelling evidence” that he had offered to pay for a class A drug, as well as paid-for sex while posing as a machine repair man called Jim.

Inconsistencies then started to appear in other rulings, prompting Tories to speculate as to why so many Brexiteers appeared to be in the firing line. In the 2020-21 period, probes were launched into the alleged wrongdoing of 13 Conservatives compared to just five Labour MPs. Of the Tories facing probes in the past year, all but three voted to leave the EU. 

In 2018, the commissioner launched an investigation into Conservative MP Craig Mackinlay, a prominent leaver, for failing to declare a company shareholding.

The company in question had been set up to create a low-cost airline but the project had been abandoned after the September 11 terrorist attacks in 2001. There were no employees or assets and the company had no bank account and capital of 2p. Ms Stone insisted on a public apology from Mr Mackinlay in the Commons, even though he appeared to have committed no more than a technical breach.

In January, the commissioner ordered Mark Francois, the chairman of the European Research Group of Brexiteer backbenchers, to apologise not once but twice for writing a letter to Michel Barnier on Commons letterhead in the summer of 2020. Yet when Bambos Charalambous, the Labour MP, was investigated for the same offence that year, he was cleared for sending a letter on Commons notepaper to deal with a claim made against him by a former landlord.

Critics have also complained about her ruling this month that Stephen Doughty, another Labour MP, did not break the code in asking a vulnerable constituent to supply him with the class C prescription drug diazepam.

The MPs’ code states that members “have a duty to uphold the law”. It is illegal to obtain diazepam – a tranquilliser – without a prescription, which Mr Doughty acknowledged. Yet Ms Stone found that while the MP had shown a “severe error of judgment”, he had “learned a very difficult lesson”.