A Labour MP has filed a formal complaint against aides to party leader Jeremy Corbyn and shadow chancellor John McDonnell for a "breach of parliamentary privilege", after allegedly entering her House of Commons office without permission.
Seema Malhotra, who quit the shadow cabinet last month in protest against Mr Corbyn's leadership, said her staff had felt "harassed, intimidated and insecure" after the unauthorised visit.
A spokesman for Mr Corbyn played down the incident and denied any intimidation had taken place.
In a statement to The Observer, Ms Malhotra said she had lodged a complaint with Commons Speaker John Bercow and had asked for an investigation.
The claims come after repeated and numerous allegations of threats, bullying and abuse against critics of the Labour leader, particularly female MPs.
In her letter to Mr Bercow, Ms Malhotra names Mr Corbyn's office manager Karie Murphy as one of those who had entered her office without permission, and said she had been "aggressive and intimidating" to staff there.
Speaking about the alleged incident on Sunday, Ms Malhotra said she was taking a stand because "this is essentially about us having confidence as members of parliament and our staff that our offices are secure".
A spokesman for Mr Corbyn said the accusation that Ms Murphy had intimidated anyone was "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," he said.
"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."
Meanwhile, in an interview on the BBC's Andrew Marr show, John McDonnell said the office manager concerned had been checking whether Ms Malhotra had moved out of the office.
"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
He also appealed for party unity and urged critics of himself and Mr Corbyn to confront them directly and not to "pick on" staff who were not in a position to defend themselves.
"We have got to stop this now. There is a small group out there that are willing to destroy our party just to remove Jeremy Corbyn. We have got to stop them," he said.
"If you want to come for me and Jeremy that's up to you, but don't pick on staff who can't defend themselves."