On 26 February 2020, after difficulty starting an Airbus A321 left engine for the first flight of the day, the same difficulty recurred on the third flight followed by subsequent en-route abnormalities affecting both engines. After no fault was found during post flight maintenance investigation, similar problems occurred starting the left engine and after takeoff from Gatwick, both engines malfunctioned and a MAYDAY return followed. The continuing Investigation has already found that the engine problems were attributable to fuel system contamination following the addition of 37 times the maximum permitted dosage of Kathon biocide during prior scheduled maintenance work.