Five-time Tour de France winner Lance Armstrong and Mario Cipollini will be unable to compete next season unless their teams are given the green light.
Armstrong has set his sights on a record sixth Tour win
Cycling's governing body, the International Cycling Union (UCI), has not yet registered either the US Postal Service or Domina Vacanze teams.
"These teams have been invited to present explanations on certain points in their files by 8 January 2004," the UCI said in a statement.
"Otherwise they won't be registered."
US Postal's Armstrong won a fifth consecutive Tour de France in 2003,
while sprint star Cipollini equalled a 70-year record by clinching his 42nd
Giro d'Italia stage.
The UCI is also investigating Italian teams Alessio Bianchi and Fassa Bortolo, Spanish outfit Kelme-Costa Blanca and Portugal's Milaneza Maia.
Cycling chiefs have already handed out registrations for 24 teams next season.
Germany's Team Coast were suspended by the UCI last May because they could not pay their riders after getting into financial difficulties.
Cipollini was handed a 3,000 Swiss francs (£1,350) fine by the UCI on Tuesday for unsportsmanlike behavior.
The UCI said he threw two water bottles at a French radio crew during the Ghent-Wevelgem race in Belgium in April.
Cipollini was wearing the rainbow jersey of World Champion at the time of