ENR 1.13  UNLAWFUL INTERFERENCE

1. Should an aircraft in flight be subjected to unlawful interference, the pilot shall endeavour to set the transponder to modeA/3 code 75 (or 7500) to give indication of the situation unless circumstances warrant the use of mode A/3 code 77 (or 7700).
2. When a pilot has selected mode A/3 code 75 (or 7500) and is subsequently requested to confirm his code by ATC he shall, according to circumstances, either confirm this or not reply at all.

Note: The absence of a reply from the pilot will be taken by ATC as an indication that the use of code 75 (or 7500) is not due to an inadvertent false code selection.