Motorsport chief Max Mosley has upped the stakes in the battle for Formula One's future by asking rebel teams to commit to the sport next month.
Mosley is determined to make F1 more competitive
At a meeting on 22 March, Mosley will propose teams sign up for 2008 within 10 days or risk losing their place.
Renault, Honda, Toyota, BMW-Sauber and McLaren-Mercedes have threatened to set up a rival series after 2007.
They want a bigger share of revenues, more say in F1's future and are upset at plans to limit technology from '08.
Mosley, president of the sport's governing body the FIA, wants to give independent teams without support from car manufacturers more of a chance by limiting the budget needed to be competitive to around £57m a year.
Ferrari's budget, the biggest in F1, is reputed to be as much as £287m, with Toyota not far behind.
"From 2008, nobody has either the right or an obligation to enter the championship," said Mosley, president of the sport's governing body, the FIA.
"It makes sense to open entries soon and then to close them again quickly. Then we can discuss refinements to the regulations before the deadline of the end of June.
"It would not surprise me to see some defections (from the five Grand Prix Manufacturers' Association teams) next month when we open the entries.
"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.
"I hope everybody will be sensible, see these are sensible rules and everyone will enter.
"There will be provision in the rules saying we could accept a late entry but you can only do that if there is a vacancy.
"We are going to stick to 12 teams because of safety. There are six already in. There are three, possibly four, (teams) talking about entering and there are five GPMA teams."
Mosley's proposal is the latest and most dramatic of a series of salvos in his battle with the car manufacturers in F1.
Last week, he proposed that they receive no money at all from the sport's revenues because his new rules would save them more than they were trying to gain from a new commercial agreement.
Mosley is to put his proposal to a meeting of the FIA World Motor Sport Council, F1's legislative body, on 22 March.
If it accepts the plan, entries for 2008 would be opened the following day and stay open for just 10 days.
Mosley said the regulations for entry would stay the same, except the £27m bond would be waived.
Ferrari, Williams, Midland, Red Bull, Scuderia Toro Rosso and Super Aguri have committed to the sport post-2007.