The aircraft was en 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 #FR212 London/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."

       Squawking alert code 7700 is used to indicate a general, but unspecified, emergency.

       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.

       On Friday August 6 a British Airways (BA) flight issued a 7700 alert code, whilst travelling from Edinburgh to London Heathrow.

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       In a statement BA said its aircraft experienced a “minor technical issue”.

       The plane had been due to land at Heathrow at 11.33am.

       However, after the emergency code was issued, this was brought forward and it landed at 10.17am. In a statement BA said: “Our pilots requested a priority landing into Heathrow, as a precaution due to a minor technical issue.

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       “The aircraft landed safely.”

       A squawk code is a four-digit number used to aid fast communication between Air Traffic Control and the aircraft.

       Each of the codes begins with a number between zero and seven.

       The squawk 7700 code sends an immediate signal to all nearby watchtowers.

       Other notable codes include a squawk 7600, which is used to inform Air Traffic Control that a plane has lost communications with the tower.

       A squawk 7500 alert is used in a hijacking and informs the tower that the plane requires emergency support from security services.