Labour MP Eric Joyce was in police custody tonight after being arrested over an alleged late night assault in a House of Commons bar.
The former soldier was arrested by Scotland Yard officers after allegations he head-butted Conservative Stuart Andrew shortly before closing time last night in the Strangers Bar, Parliament's pub for MPs.
Mr Andrew, MP for Pudsey, who has no visible cuts or bruises, responded to concerns about his welfare on Twitter by writing: "I'm ok."
Labour suspended Mr Joyce from the party pending the results of the police investigation, branding the alleged incident "extremely serious", though he remains MP for Falkirk.
The Metropolitan Police confirmed officers detained a man in his 50s after being called to reports of an incident at a bar within the Palace of Westminster at around 10.50pm.
He was taken into custody at a central London police station and remains there tonight.
A Labour Party spokesman said: "This is an extremely serious incident. We have suspended Eric Joyce pending the results of the police investigation."
Speculation about the incident has been rife in Westminster but Commons Speaker John Bercow thwarted any attempt to raise the matter in the chamber by issuing a stern warning at the start of proceedings.
He told MPs: "Members will be aware of reports of a serious incident in the House last night.
"I have been informed by the Sergeant at Arms that the honourable member for Falkirk has been detained in police custody.
"The matter is being investigated. I take this matter very seriously, as do the House authorities. I would ask that no further reference should be made to these reports in the Chamber today."
Mr Joyce, 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.
A Scotland Yard spokesman said: "We were called at approximately 10.50pm last night 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.
"The man remains in custody at a central London police station."