Teams threatening to break away from Formula One risk losing their place on the grid if they do not take advantage of a 10-day window of opportunity next month to commit to the sport.
Max Mosley, president of the sport's world governing body (FIA), told reporters he hoped the five 'rebel' manufacturers would announce they will stay with Formula One after the sport's Concorde Agreement runs out at the end of 2007.
However, he warned Renault, Honda, Toyota, McLaren-Mercedes and BMW-Sauber they would not be guaranteed one of the 12 team slots if they failed to sign up in time.
"From 2008, nobody has either a right or an obligation to enter the championship," Mosley told reporters.
Six teams - Ferrari, Williams, Midland F1, Red Bull, Scuderia Toro Rosso and Super Aguri - have committed to F1 post-2007. Others have said they are prepared to join Formula One if costs are slashed.
"We are going to stick to 12 teams because of safety and all the pits now are arranged for 12," Mosley said.
"There are six in there, there are three possibly four people talking about entering and there are five GPMA (Grand Prix Manufacturers' Association) teams.
"It makes sense to open entries soon and then to close them again quickly. Then we can discuss with the people that have entered refinements to the regulations before the deadline of the end of June."
Mosley's plans to reduce costs to attract smaller, independent teams will be discussed at a meeting of the World Motor Sport Council in Paris on March 22.
Entry to the 2008 championship will open the following day and will stay open for 10 days to allow time for discussions before the deadline for regulations at the end of June.
"The regulations for entry are identical to the present day except that we have deleted the $48million bond," said Mosley.
"We would look at their factory, their business and be satisfied that they are capable of doing what we wanted.
"It would not surprise me to see some defections (from the GPMA) next month when we open the entries. I hope everybody will be sensible and see these are sensible rules and everyone will enter.
"I have got a real determination to see this through. I honestly believe that if F1 is to prosper then we have got to get the costs down so that the independent teams can survive."
