It is alleged that Mr Wareing's company received money from the Serbian firm in return for his lobbying activities.
Mr Wareing, who now faces an investigation by Sir Gordon Downey, the Parliamentary Commissioner for Standards, has had the party whip suspended until the inquiry is completed. He has "welcomed" the investigation but has not denied the allegations.
Mr Wareing has long had an interest in the former Yugoslavia and a significant proportions of the questions he asked in Parliament concerned matters relating to the war there and its aftermath.
He was criticised in 1995 for holding a meeting with Bosnian Serb leader Radovan Karadzic and with General Ratko Mladic, who were both accused of being responsible for ethnic cleansing.
Last year he asked a series of questions on whether various ministers would be visiting Serbia and whether the Government would be trying to promote British trading interests in the former Yugoslavia. He also asked a question about whether the Export Guarantee Credit scheme covered Serbia and he was told that it did not.
Mr Wareing has long been seen as pro-Serbian, though he denied in Parliament that he was a supporter of the Milosevic regime. If his link with the Serbian company is proved, Mr Wareing is likely to face much more than the minimum punishment of a reprimand and at best will receive a suspension from the House or, at worst, possible expulsion.Reuse content