Boris Johnson has been accused of repeatedly lying to parliament – both over Downing Street parties, and over claims where he tried to link opposition leader Keir Starmer to notorious sex offender Jimmy Savile.
Despite rules in the ministerial code saying that members of the government should resign if they mislead parliament, in practice Johnson is likely safe from an investigation.
This is because under the UK government's weak oversight rules, only the prime minister has the power order investigations under the ministerial code. This includes when the investigation is into himself.
The ministerial code says that ministers must “correct any inadvertent error at the earliest opportunity”.
But if members of the government are found to have lied to parliament deliberately they can face a formal inquiry under the code.
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However, this power is reserved to the prime minister, who oversees the ministerial code.
"The problem with this is that only the prime minister can decide to open an inquiry into whether a minister has breached the Ministerial Code," says Dr Alice Lilly, a senior researcher at the Institute for Government.
"And it is also up to the prime minister to decide on the sanctions that any minister who has breached the Code may face."
In the past Mr Johnson has been reticent to recommend serious sanctions against other ministers who broke the code. In 2020 the prime minister's senior ethics advisor Alex Allan, found that Home Secretary Priti Patel had broken the ministerial code by bullying staff.
But Mr Johnson said that he did not believe Ms Patel was a bully and had “full confidence” in her. The ethics advisor subsequently resigned from the role.
But is there another way in which the prime minister could be held to account?
Dr Lilley notes that "ministers are also bound by the Nolan Principles of Public Life, which state that 'holders of public office should be truthful'".
"But these are principles, not rules," she says, noting that the remit of the Parliamentary Commissioner for Standards who might apply the code also "does not extend to remarks made in the Commons".
MPs could also table a substantive motion to censure the prime minister, she says. This happened in 1963 during the Profumo affair, where the then war secretary was judged by MPs to have treated parliament with “grave contempt” for making a statement in the Commons “containing words which he later admitted not to be true".
"But ultimately, the outcome of any vote on whether a minister has committed a contempt by making a false statement is likely to depend on the size of the government’s majority. So that is also a limited mechanism," she adds.
Ultimately, there is unlikely to be a workable legal shortcut to ousting the prime minister for lying if he decides not to investigate himself.
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Political pressure for the PM to resign would be the final safeguard. This would have to wait for a general election, unless the Conservative party internally decides that Mr Johnson is too much of a political liability. This process is playing out in Westminster currently, and it remains to be seen when or whether the Conservatives might oust him.
The Council of Europe's equalities watchdog has previously raised concerns about the UK's weak oversight systems.
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In its latest report in 2021 the council’s corruption monitoring arm GRECO said just five out of 12 recommendations previously handed to the UK government in 2018 had been “dealt satisfactorily with”.
It warned in particular about the lack of independence of the UK government's standards watchdogs covering government ministers.