Eric Joyce has also been suspended by his party, The Independent reported. He was arrested late Wednesday in the Strangers Bar.
The Strangers Bar, sometimes nicknamed the Stranglers Bar for its supposed rowdiness, has a reputation as a hangout for Labor MPs, The Independent said. Witnesses said Joyce head-butted Stuart Andrew after yelling "There are too many f--king Tories in here."
Phil Wilson, a Labor whip, was injured trying to restrain Joyce, witnesses said.
"This is an extremely serious incident," the Labor Party said in a statement. "We have suspended Eric Joyce pending the results of the police investigation."
The Independent suggested that more Conservatives have been spending time in the Strangers Bar because the party's success in the 2010 elections has meant an overflow crowd in their stronghold, the Smoking Room.
Joyce, 52, was elected to Parliament in 2000 from a Scottish constituency after leaving the army. He resigned as Labor's Northern Ireland spokesman after failing a breath test during a 2010 traffic stop.
He has a court date March 7.