One of the busiest bars has been closed after a fight between two staffers left one injured in hospital.
rushed to a courtyard near the Sports and Social Club Bar at 6.30pm on Tuesday.
One of the men, aged 57, was arrested on suspicion of causing grievous bodily harm and affray.
He was taken to a central London police station.
A second man, aged 64, suffered non-life threatening injuries and was taken to hospital by paramedics, Scotland Yard said.
No other people were involved.
A House of Lords spokesman said: "Following an incident on the parliamentary estate last night involving two parliamentary staff, after leaving the Sports and Social Club Bar, an investigation is under way and the bar will be temporarily closed until that investigation is complete."
Officers from Westminster are investigating the incident.