A man named by sources as serving Labour MP Eric Joyce has been arrested on suspicion of assault following a disturbance at the Palace of Westminster.
Scotland Yard confirmed officers detained a man in his 50s after being called to reports of an incident at a bar within the House of Commons at around 10.50pm on Wednesday.
He is currently being held in custody at a central London police station, the Met said.
A Scotland Yard spokesman said: "We were called at approximately 10.50pm (on Wednesday) to reports of a disturbance at a bar within the House of Commons. A man aged in his 50s was arrested by officers on suspicion of assault. He remains in custody in a central London police station. Inquiries are continuing."
Mr Joyce, the MP for Falkirk since December 2000, served in the Army Education Corps before pursuing a career in politics.
In 2010 he resigned as shadow Northern Ireland minister after pleading guilty to failing to provide a breath test.
From 2003 Mr Joyce served as a Parliamentary Private Secretary (PPS) to a number of government ministers.
Mr Joyce quit as the PPS to Bob Ainsworth in 2009 due to his concerns over the war in Afghanistan. Prior to that he had been a parliamentary aide to John Hutton, including the period when he was defence secretary, Mike O'Brien and Margaret Hodge.
According to his constituency webpage, Mr Joyce has a constituency office in Denny and an office in Portcullis House, Westminster.
During his time in Westminster he has held an interest in defence and military issues due to his army background, the website adds.