Bastia's midfielder Florian Thauvin bagged a brace as Lyon's Ligue 1 title hopes took a hammer blow at the Stade Armand Cesari.
Thauvin, who is remaining with Bastia for the rest of the season having signed for Lille in January, put the hosts in front just before half-time although Lisandro Lopez levelled in the 53rd minute.
That was as good as it got for the visitors, who fell behind almost immediately afterwards through Anthony Modeste's strike while Thauvin's second on the hour left them with a mountain to climb.
Wahbi Khazri ensured there was no way back as Lyon, who could trail Paris St Germain by seven points if the league leaders beat St Etienne tomorrow, suffered their heaviest defeat of the season.
It was the hosts who started brightly and Khazri came within inches of breaking the deadlock after firing wide from the edge of the area, while Alexandre Lacazette volleyed wide at the other end.
There were precious few chances for either side in a quiet opening half although Thauvin brought the game to life a minute before half-time.
The hosts caught Lyon cold on the counter and Khazri played through Thauvin, who drove low past goalkeeper Remy Vercoutre.
Bastia's strike seemed to sting Les Gones into action and Argentinian Lisandro levelled matters eight minutes after the break when he converted from Yoann Gourcuff's free-kick on the right.
But anyone thinking that would be the catalyst for Lyon to go on to victory were proven wrong when Modeste put Bastia in front once more two minutes later.
Khazri collected the ball from goalkeeper Mickael Landreau's long clearance and dribbled into the area before cutting the ball back to Modeste, who drilled past Vercoutre.
And Bastia were well on their way to the points when Thauvin bagged his second after latching on to Modeste's through ball and slotting across Vercoutre.
The visitors could not get themselves back into the game and they nearly fell further behind, with Vercoutre stopping Modeste, who had burst clean through on goal, while Claudio Beauvue's 25-yard free-kick cannoned off the post.
Bastia were not to be denied, however, and Tunisia international Khazri added their fourth with a delightful curling effort into the top corner.