The French Open will move from its scheduled spring slot to September because of the coronavirus pandemic.
The year's second tennis Grand Slam was due to begin on May 24 but will now run from September 20 to October 4.
An FFT statement read: "The whole world is affected by the public health crisis connected with Covid-19.
"In order to ensure the health and safety of everyone involved in organising the tournament, the French Tennis Federation has made the decision to hold the 2020 edition of Roland Garros from September 20 to October 4, 2020.
"The containment measures in force make it impossible to prepare and therefore organise it on the dates initially planned.
"In fact, to preserve the health of employees, service providers and suppliers in a responsible manner during the preparation period, the FFT have decided to adopt the only scenario which will reconcile the organisation of the tournament in 2020 and the fight against Covid-19."
A roof has been installed over Roland Garros' centre court, Philippe Chatrier, with floodlights on the three other main courts, enabling play to go on later and therefore making the autumn dates possible.
With regular tournaments suspended until the end of April, it was looking increasingly unlikely that the French Open would be able to be staged as planned.
The news means the tournament is due to begin a week after the end of the US Open, presenting a challenge for players to reacclimatise to clay and a different time zone at short notice.
It will also require a reorganisation of the ATP and WTA seasons, with major women's tournaments in Tokyo and Wuhan due to take place during that fortnight.
Any tickets already purchased for the French Open will either be refunded or exchanged for the new dates.
Tennis players greeted the news with shock on social media.
Canadian Vasek Pospisil, a vocal member of the ATP Player Council and an advocate for a players' union, was the most outspoken.
He wrote on Twitter: "This is madness. Major announcement by Roland Garros changing the dates to one week after the US Open. No communication with the players or the ATP. We have ZERO say in this sport. It's time. #UniteThePlayers".
Meanwhile, former Roland Garros and current Wimbledon champion Simona Halep has announced she is donating money to buy medical equipment in her home country of Romania to help manage the crisis.
Elsewhere, promoter Eddie Hearn has admitted that Anthony Joshua's next world heavyweight title defence against Kubrat Pulev could be pushed back a month to July.
Joshua is scheduled to face the Bulgarian at Tottenham's new stadium on June 20, but that date could now be required by the club for a potential extension of the Premier League campaign.
Hearn, whose Matchroom organisation earlier confirmed the postponement of all boxing events in March and April, once again dismissed the prospect of any of his major bouts being staged behind closed doors.
Referring to the June 20 date, Hearn said: "At the moment it's still in place. Obviously with the news of the Euros being cancelled (Euro 2020 has been postponed until next year) and the possible extension of the Premier League season, Spurs may need that stadium in June.
"We do have an option to run that fight in July as well at the same stadium. Everything now is really a case-by-case situation and a day-by-day situation of trying to think on your feet.
"June 20 is a long time, it's in our plans and we have been speaking to Tottenham to make sure we're on the same page and that we know we have the same strategy moving forward."
Prior to Joshua's bout with Pulev, Dillian Whyte is scheduled to meet Alexander Povetkin on May 2 and Dereck Chisora is set to face Oleksandr Usyk on May 23 - both bouts which Hearn says are "subject to change".
The London show featuring Josh Kelly's European title clash with David Avanesyan has been cancelled and all bouts will be subsumed into future cards.
The April 4 Newcastle show, featuring Lewis Ritson versus Miguel Vazquez as well as a world title shot for Savannah Marshall, has been re-arranged for June 27, and the April 24 Doncaster bout between Terri Harper and Natasha Jonas will also take place in June.
Queensberry Promotions announced the heavyweight bout between Daniel Dubois and Joe Joyce scheduled for the O2 Arena on April 11 has now been moved back to July 11.
Josh Taylor's world title fight scheduled for May 2 in Glasgow has also been postponed.