Aerad Flight Information Supplement ATC 43:
TRACK AND ETA
Adherence to Flight Plan
12.2.3. Change of True Airspeed (TAS).
(a) If at any time a change in TAS of five percent of that given in the Flight Plan, is premeditated, with the intention that the revised TAS is to be maintained for the remainder of the flight or portion thereof, the pilot must notify the appropriate Oceanic Centre before making the change or as soon as possible thereafter.
(b) Pilots will adhere to the true Mach number approved by ATC and shall request ATC approval before making any change thereto. If essential to make an immediate temporary change in the Mach number (e.g. due to turbulence), ATC shall be notified as soon as possible that such a change has been made.
(c) ATC will asume that the last assigned MACH number will be maintained in CRUISE Flight and in any climbs or descents made in the course of the Flight, unless advised by the pilot concerned.