John Bercow is among prominent MPs who could come under investigation over bullying claims after the government paved the way for the Commons complaints system to include historical allegations.
The Commons speaker, who is the subject of bullying allegations from former staff, is the most high-profile MP accused of wrongdoing and among the most likely to face an inquiry if the current process is changed.
Mr Bercow is accused by several staff members of intimidating and threatening behaviour. He has strongly denied the allegations, and last year the Commons standards committee blocked an investigation into the claims.
Others who could now face investigations include Labour MPs Keith Vaz, who was accused of bullying a former Commons clerk. He also denied the claim.
MPs will vote next week on whether the current system, which only covers allegations since June 2017, should be extended further back.
The move comes after an independent inquiry into bullying in the Commons found "unacceptable" levels of abuse and harassment that amounted to "a significant problem".
The report, by Gemma White QC, urged Commons authorities to open up the current complaints procedure, introduced in 2017, to include alleged past. The change was also recommended by a similar report, by Dame Laura Cox, last year, but is yet to be implemented.
Theresa May is understood to want to push the change through in her last days as prime minister, as part of her attempts to salvage a legacy.
Ms White said she agreed "entirely" with Dame Laura that the June 2017 cut-off date should be removed and said there would be "no proper reason" for not doing so. She also called for a ban on former employees making formal complaints to be lifted.
Referring to next week's vote on changing the rules, she said: "If [MPs] do not vote in favour of the resolution, confidence in the ICGS [the Commons' independent complaints and grievance scheme] will be seriously undermined. The length of time taken to put into effect a straightforward amendment...has already led some to question the House’s stated desire to address the now widely accepted problems of bullying and harassment."
She said that just 34 of the 650 MPs have signed up for a course to enforce a new “behaviour code”, introduced a year ago, while helplines set up last year to deal with complaints of harassment and sexual misconduct since 2017 have received some 783 calls and emails, leading to as many as 42 investigations being opened.
The inquiry found that MPs routinely “shout at, demean, belittle and humiliate their staff on a regular basis, often in public”.
Ms White warned: “The constant ‘drip, drip’, as more than one contributor put it, eats away at the employee’s self-confidence until they become anxious, exhausted and ill, incapable of performing their job and (often following a period of sick leave) resign or are dismissed.
“Well over half of the people who contributed to this inquiry described suffering significant mental and/or physical illness as a result of this type of bullying behaviour."
A spokesperson for Mr Bercow said: “The speaker welcomes the expansion of this scheme to include non-recent cases as another positive step towards changing the culture of parliament.”