* Porsche to join electric series in 2019
* Le Mans champions to quit top tier of World Endurance racing
* Will continue to race at Le Mans in GT category
* Mercedes, Audi and BMW already committed to Formula E (Adds details, reaction)
FRANKFURT, July 28 (Reuters) - Porsche announced on Friday it was leaving the top category of Le Mans sportscar racing at the end of the season to join German automotive rivals in the Formula E electric championship in 2019.
"Entering Formula E and achieving success in this category are the logical outcomes of our Mission E," Development Chief Michael Steiner said in a statement.
Porsche, whose factory team have been Le Mans champions for the last three years and won the World Endurance Championship (WEC) in 2015 and 2016, has already taken the first steps towards developing its own Formula E racing car this year, the group said.
The announcement had been widely expected in the motorsport world and came after Mercedes confirmed on Monday its move into Formula E and exit from the German Touring Car (DTM) championship.
Audi, like Porsche also part of the Volkswagen (DE:VOWG_p) Group, announced last October that it was quitting Le Mans and the WEC to shift resources to Formula E.
Other manufacturers already involved in the electric series, whose third season ends this weekend in Canada, include German rivals BMW as well as Renault (PA:RENA), Jaguar, Citroen and India's Mahindra.
The arrival of Mercedes and Porsche in the series for season six, which starts in 2019 and ends in 2020, will take to 12 the number of Formula E teams.
Porsche's exit from the main LMP1 Le Mans category will be a big blow for the WEC, which risks being left with Toyota as the only major car manufacturer competing at that level -- hardly viable for a championship.
There was no immediate comment from series organisers but Jean Todt, the head of the International Automobile Federation (FIA) expressed mixed feelings.
"I'm very happy that Porsche is coming to Formula E, but I regret their decision to leave the World Endurance Championship," the Frenchman said in a statement.
Both the WEC and Le Mans have multiple tiers of entries and Porsche will continue to compete in the GT class with its 911 RSR model.
The French race is the crown jewel in the FIA-sanctioned WEC but stands apart as a highlight of the motorsport calendar, with some 260,000 spectators attending this year's annual sportscar festival.
Porsche said it will keep the LMP1 team fully intact, including the factory drivers.
Formula E chief executive Alejandro Agag hailed the continued growth of his series.
"If somebody told me when we started this project five years ago, that we'd be announcing a partnership with a brand like Porsche, I wouldn't have believed it," said the Spaniard.