From JAR-OPS1: Appendix to 1.375
[(1)] If, as a result of an in-flight fuel
check, the expected fuel remaining on arrival at
the destination is less than the required alternate
fuel plus final reserve fuel, the commander must
take into account the traffic and the operational
conditions prevailing at the destination
aerodrome, along the diversion route to an
alternate aerodrome and at the destination
alternate aerodrome, when deciding [whether] to
proceed to the destination aerodrome or to divert,
so as to land with not less than final reserve fuel.