You should never EVER place a requirement other than UNIQUE (and perhaps the fact that it is a LONG INTEGER, when establishing relationships to foreign keys) on an AUTONUMBER field.
If the number MUST have the property of MONOTONICALLY increasing, it cannot be an autonumber.
Rethink your requirements. Search this forum for "Autonumber" topics to see how often this comes up - and how often we have to guide folks away from abuse of autonumbers.