Iberia claim Madrid-Barcelona is the busiest in Europe!
Madrid Airport-Barcelona Airport is the busiest air route in Europe. However Dublin-London(LTN, LGW, STN, LHR, LCY) is the busiest city pairing in Europe and the second busiest in the world. There are currently 51 flights per day to the 5 London airports. LHR-20, LGW-13, STN-10, LTN-4, LCY-4