Labour has launched an investigation into Birmingham MP Gisela Stuart following an angry meeting of backbench MPs last night.
A disciplinary hearing will be held tomorrow after fellow MPs accused her of helping the Conservatives.
Geoff Hoon, Labour's the Chief Whip, will ask a committee of backbenchers to look into the campaign for a referendum on the controversial European Treaty, which Ms Stuart (Lab Edgbaston) is helping lead.
Also in the dock are Labour backbenchers Kate Hoey, Graham Stringer and Frank Field.
But Ms Stuart has vowed to continue campaigning for a referendum.
In a private meeting of Labour MPs, held in a Commons committee room last night, she insisted she was supporting Labour Party policy, because the party promised a ballot in its manifesto during the 2005 General Election.
The public would give Labour credit for keeping its promise to hold a referendum, she said - and punish it if it went back on its word.
Mr Hoon, who is responsible for enforcing discipline in the party, delivered an angry speech accusing the MPs of undermining their own colleagues.
Some of Ms Stuart's colleagues are angry about the tactics used by the I Want a Referendum campaign.
The campaign has organised unofficial referendums in marginal constituencies held by high-profile politicians.
These include Labour-held Redditch, Worcestershire, where Jacqui Smith, the Home Secretary, will defend a majority of 2,716 over the Conservatives at the next election.
Ms Stuart's critics claim the campaign will undermine Ms Smith, and could help the Conservatives in the constituency.
The campaigners also launched a referendum in the East Renfrewshire constituency of Europe Minister Jim Murphy yesterday.
Mr Hoon has asked Labour's Parliamentary Committee, which includes senior backbenchers, to examine the complaints.
This is the same body which launched an inquiry into Ladywood MP Clare Short in 2006, which eventually led to Ms Short resigning from the Parliamentary Labour Party.
In theory, the Parliamentary Committee can recommend that the whip is withdrawn from rebel MPs, effectively kicking them out of the party.
Ms Stuart last night denied she was campaigning against Labour colleagues. She said: "I am campaigning in support of Labour policy."