A Labour MP in England has accused aides to Jeremy Corbyn and shadow chancellor John McDonnell of entering her House of Commons office without permission.
Seema Malhotra said the action constituted a "breach of parliamentary privilege" and has lodged a formal complaint with Commons Speaker John Bercow.
In a statement to The Observer, Ms Malhotra - who quit the shadow cabinet last month in protest at Mr Corbyn's leadership and is now backing the challenger Owen Smith - said her staff felt "harassed, intimidated and insecure" as a result of what had happened.
A spokesman for Mr Corbyn played down the incident and said that claims of intimidation were "untrue".
The latest extraordinary twist comes against repeated claims that critics of the Labour leader - particularly women MPs - have faced threats, bullying and abuse if they speak out.
In her statement, Ms Malhotra said: "I have discovered that members of staff working for John McDonnell and Jeremy Corbyn have gained unauthorised entry into my office in parliament.
"The implications of this are extremely serious. This is a breach of parliamentary privilege and is a violation of the privacy, security and confidentiality of a member of parliament's office.
"Furthermore, my staff, including an intern, who have always been courteous and open, have felt harassed, intimidated and insecure and decided themselves it would be best to not leave anyone alone in the office.
"I have made a formal complaint to the Speaker of the Commons and to Jeremy Corbyn MP, and requested an investigation into how this could have happened."
In her letter to Mr Bercow, quoted by The Observer, Ms Malhotra names Mr Corbyn's office manager, Karie Murphy, as one of the aides involved, and accuses her of being "aggressive and intimidating" towards the staff involved.
A spokesman for Mr Corbyn said: "The accusation that Karie intimidated anyone is untrue.
"As an office manager on the Leader of the Opposition's floor, Karie has a key to open all offices. She accessed the office in question to confirm when it would be vacated.
"It is a month since Seema Malhotra resigned as shadow chief secretary to the Treasury, and the office is intended for the person holding that position."
Ms Malhotra said the unauthorised entry by staff into her office constituted a serious breach of parliamentary privilege.
"The implications of this are extremely serious. This is a breach of parliamentary privilege and is a violation of the privacy, security and confidentiality of a member of parliament's office," she told The Observer.
"Furthermore, my staff, including an intern, who have always been courteous and open, have felt harassed, intimidated and insecure and decided themselves it would be best to not leave anyone alone in the office."
However Mr McDonnell said the office manager concerned had simply been checking whether Ms Malhotra had moved out of the office after quitting as shadow chief Treasury secretary last month in protest at Mr Corbyn's leadership.
"I have now got a member of staff - she's a widow with daughters, this is her sole income - she's now worried she's going to lose her job and face prosecution because it's described as a break-in. That's just so distressing, it's unacceptable. This has obviously been an error," he said.
But with the rift between leadership and the party's MPs showing no signs of ending, Mr McDonnell acknowledged that there would have to be some form of mediation once the current leadership contest is over.
"What we should do is do some mediated negotiations between the Parliamentary Labour Party, the NEC (national executive committee) and others so we use the leadership contest to discuss the issues but also we use it to help heal some of the wounds and bring it together," he said.