The ban is backdated though, to 20 November, as his lawyers had hoped, so that he will still be free to ride in the Epsom Derby on June 1, though he will miss the prestigious English Guineas meeting.
The French governing body has never before backdated a ban.
The Italian-born jockey, who is based in Britain, tested positive for a non-performance-enhancing banned substance at the Prix de l’Arc de Triomphe trials on 16 September at Longchamp in France.
Dettori succeeded in getting back in time for the Derby because France Galop took into account his decision not to ride at all after his medical certificate was withdrawn by the Medical Commission on November 20.
However Dettori must return to France for more medical tests before he can be cleared to ride again.
"He must return to France and undergo biological tests on April 20. If they are clear then he is free to resume riding on May 19," said a spokesman for France Galop.
Dettori's lawyer Christopher Stewart-Moore issued a statement saying that Dettori was contrite and felt he “had let down the sport he loves and all those associated with it, as well as the wider public.”
In line with its policy on doping, France Galop has not revealed the nature of the banned substance.
Dettori was one of five jockeys tested at Longchamp on September 16 but the French governing body insist he was not targeted because of a tip-off.
Dettori has had a difficult year. His hugely successful 18-year partnership with the Dubai-based Godolphin operation came to an acrimonious end in October. Relations between the two parties had deteriorated during the season where he was increasingly playing second fiddle to French rider Mickael Barzalona.
The partnership broke down when he rode Camelot, owned by Godolphin's bitter rivals in Ireland, the Coolmore Stud, for Europe's most prestigious race, the Prix de l'Arc de
Triomphe, in October.
He has won over 100 Group One races, including an Epsom Derby and also three Arcs.