The final three teams have been added to the grid for the first A1 Grand Prix, which will take place at Brands Hatch on Sunday.
Pakistan is among the countries which will take part in the A1 series
Austria, Italy and Japan complete the 25-team line-up.
Austria's entry will be managed by Red Bull boss Dietrich Mateschitz and former world champion Niki Lauda, whose son Mathias will do the driving.
"I'm pleased to welcome three countries with such a strong pedigree," said A1 founder Sheikh Maktoum Hasher Maktoum.
"We now have 25 countries battling for the honour of being the best in the world, in a competition that starts in only a few short days."
The Italian squad will be run by ex-grand prix driver Piercarlo Ghinzani, who is not expected to name his driver until Thursday.
Ryo Fukuda will drive for Japan in a team led by ex-Formula One back-marker Hideki Noda.