The framework for next week's rejigged Young Driver Test at Silverstone has been confirmed by the FIA.
Last week, Formula 1's governing body opened up the July 17-19 test to race drivers in order to allow them to test the new Pirelli tyres set to be introduced at the Hungary Grand Prix.
While it was originally suggested that the test might be extended to a fourth day if required, the FIA ruled that out on Tuesday.
It also made it clear that race drivers must focus solely on tyre testing for the duration of the three days.
"Only the 2012 construction will be used for the test but with the hard, medium and soft 2013 compounds," an FIA statement read.
"The test will now allow teams to field drivers who have competed in more than two F1 World Championship events provided that the purpose of them doing this is to test tyres for Pirelli.
"In order to meet this requirement, all teams shall ensure that any changes made to a car at these times are exclusively related to the tyre tests set out in the run plan provided by Pirelli.
"An FIA observer will be appointed to ensure the regulations are being followed."
It was agreed by the various teams last week that race drivers would only be allowed one day of testing each, in order to allow inexperienced drivers time in the seat as well.
Mercedes will not play a part in the test, a result of the ban imposed on them by the International Tribunal last month for their part in 'Testgate'.