On 12 December 2009, an Airbus A340-600 being operated by Virgin Atlantic Airways on a scheduled passenger flight departing from London Heathrow in night VMC was slow to rotate and the aircraft settled at an initial climb speed below VLS - defined as the lowest selectable speed which provides an appropriate margin above the stall speed. This prompted the PF to reduce the aircraft pitch attitude in order to accelerate which resulted in a poor rate of climb of between 500 and 600 fpm. The flaps were retracted on schedule and the aircraft continued its climb. At no time was full takeoff thrust selected. Later in the climb, the crew looked again at the take off data calculation and realised that they had made the departure with insufficient thrust set and using Vr and V2 speeds which were too low for the actual aircraft weight. The flight to the planned destination was completed.