The carrier, which flies from Heathrow, Gatwick, Edinburgh and Dublin, was due to ground the majority of its fleet from 10am Friday UK time but has already removed some services to avoid displacement of crews and aircraft.
Its chief executive Alexis von Hoensbroech said an impasse had been reached in talks over pay, adding he had been left “with no choice but to begin taking the painful steps of preparing for the reality of a work stoppage”.
The carrier is permitting changes to bookings for travel until 21 May as well as full refunds.
Action affects WestJet and its budget brand Swoop, with “the majority” of Boeing 787 aircraft, which fly transatlantic, being parked.
The airline said domestic WestJet Encore, WestJet Link and “limited” Boeing 737 flights will continue to operate.
