The aircraft was on route to Dublin in the Irish Republic.
It is not yet known what caused the diversion.
According to Airport Webcams, which monitors global aviation, the aircraft had a “defective autopilot”.
The account tweeted: “Ryanair #FR212London/Stansted to Dublin (Boeing 737-800 EI-EVO) declared Pan Pan, squawked 7700 General Emergency & just returned to STN due to a defective autopilot.
“Issued with approach vectors into STN by Essex Radar.”
On Twitter Carolyn White asked: “Why has flight FR212 STN to DUB been turned back?”
Another social media user replied: “Defective autopilot apparently”.
Ryanair has been contacted by Express.co.uk for comment.
More to follow…