Two editions of the Le Mans 24 Hours will count towards the next World Endurance Championship in what is being billed as a 'superseason' covering 2018 and the first half of 2019.
An eight-race season will climax at Le Mans in 2019 ahead of the start of a new-look WEC that will run across two calendar years, starting in October with the first of seven races and finishing with the French enduro.
A 12-hour race at Sebring running on the same weekend as the traditional IMSA SportsCar Championship race of the same duration will be part of the WEC 'superseason'.
The traditional Sebring 12 Hours IMSA race will start on the Saturday at 10am and then the WEC event will begin two hours after it finishes, starting at midnight.
The changes are part of a radical package of changes to the WEC put together by series promoter the Automobile Club de l'Ouest in conjunction with the FIA in the wake of Porsche's announcement that it will be leaving the LMP1 division at the end of this season.
The 2018/19 schedule will include a single race at Spa ahead of Le Mans, before the series resumes in October at Fuji.
There will be one more race in 2018, at Shanghai in November.
The 2019 leg of the 'superseason' will begin in February with an event that has yet to be determined.
Sebring, Spa and Le Mans will then complete the calendar.
WEC boss Gerard Neveu said: "The 2018/19 season will be a season of transition.
"Five years ago, when the WEC was created, we worked on the idea of finishing at Le Mans, but before it was not possible.
"The football world cup finishes with the final and without Le Mans, there is no WEC, so it makes sense."
Le Mans will no longer be a double-points round, though there will be bonus points for the 24 Hours and Sebring courtesy of their longer duration than the regulation six-hour races.
Neveu explained that it was important that the 'superseason' does not result in an increase in costs.
This will be made possible by the reduction in the number of races and a reduction in travel costs by shipping the cars only by sea, rather than current mixture of sea and airfreight.
The technical regulations for LMP1 will remain unchanged for the 'superseason', except for measures designed to give privateers the same chance of victory as the factory hybrids.
This means the end of the separate privateer P1 class.
ACO sporting director Vincent Beaumesnil said: "When we had three manufacturers [before Audi's withdrawal at the end of 2016] we made regulations that allowed the privateers opportunities to fight.
"Now there is only one manufacturer [presuming Toyota continues] we are going to try to bring them even closer.
"This transition period is a good window for privateers to join."
Beaumesnil also revealed that it would be possible for a manufacturer to join the WEC with a non-hybrid car.
Entrants in the 'superseason' will be required to contest all the races, which rules out the possibility of Toyota contesting a partial schedule in the future.
2018/19 WEC 'superseason' calendar
May 5 6 Hours of Spa
June 16-17 Le Mans 24 Hours
October 14 6 Hours of Fuji
November 4 6 Hours of Shanghai
February 2019 TBA
March 16 12 Hours of Sebring
May 4 6 Hours of Spa
June 15-16 Le Mans 24 Hours