Ahead of the Australian Grand Prix, Ferrari, Red Bull and Renault lodged a protest against Toyota, Williams and Brawn GP over the design of their cars' diffuser.
Although the protest was rejected by the race stewards, the teams appealed the decision and it will be again judged by the FIA on 14th April.
The Brawn GP cars dominated Sunday's race, with Jenson Button leading team-mate Rubens Barrichello to a one-two. The Toyotas finished in third and fifth, while Williams driver Nico Rosberg was seventh.
Alonso, sixth in the race, reckons it will be very hard to catch the Brawn GP cars if the FIA decides its diffuser is legal.
"We'll have to see what they decide on the 14th," Alonso told Cadena Ser radio. If they are allowed to race with it they have a bit of an advantage and all teams will try to copy that idea.
"But it's difficult because you have to work on the whole car. It's not just adding the diffuser and suddenly the car is a second quicker.
"The diffuser makes you go fast if you have a new front end, new sidepods, a new engine cover. You have to rebuild the whole car and that would take a lot of months.
"We'll see what happens. It's a complicated issue and the championship could be more or less decided on the 14th. If the diffusers are legal, the Brawns are going to be nearly unreachable for any other team."
Alonso said Brawn GP was in a league of its own at the moment and admitted it will be hard to try to beat them in the upcoming races.
"They have proved they are on a different level. Right now they are unbeatable," he added. "They were first and second in qualifying and in the race, and it's going to be hard to beat them in these first few races."