SAO PAULO, Oct 18 - Formula One teams will no longer be allowed to run three cars at grand prix practice sessions from next season, the sport's governing body announced on Wednesday.

However, test and reserve drivers will still have a chance to appear on track under a revised weekend timetable designed to give spectators more for their money.

The International Automobile Federation (FIA) said in a statement ahead of Sunday's season-ending Brazilian Grand Prix that Friday free practice would be expanded to two 90-minute sessions from two hour-long ones at present.

Teams will be limited to two cars in any one session, to be driven by a nominated third driver or the regular two race drivers.

However Friday practice will be exempt from the two-race engine rules, a move that should give fans far more to watch than at present.

Engines must last for two successive grands prix and the current rules impose a 10-place penalty on the starting grid for any driver who has an unscheduled change, unless he fails to complete a race.

As a result, many of the drivers that the fans want to see, such as Renault's world champion Fernando Alonso, do little or no driving in Friday's opening practice to save their engines for Saturday qualifying and the race.

Other rule changes for 2007, when Bridgestone will be the sole supplier, include doubling the amount of dry weather tyres available to drivers to 14 sets per weekend.

Stewards will be allowed to impose grid place penalties and safety car procedures have been tightened up to prevent lapped cars getting in the way of the leaders.

"Before a race is resumed any lapped car between cars running on the lead lap must pass those cars and the safety car and then proceed around the track to take up position at the back of the line of cars behind the safety car," the FIA said.

A global positioning system, using cockpit light displays, will supplement the traditional use of marshals waving warning flags to alert drivers.

"The system will enable Race Control to see the exact location of all cars at all times," the FIA said.

"It also will be able to communicate with the cars to illuminate cockpit lights (red, yellow and blue flags) and allow drivers to give a start abort signal if necessary."