Parker, 33, retained his interim WBO title in February with a second-round knockout win against Martin Bakole.

Ukrainian Usyk is the WBC, WBA (Super) and WBO heavyweight champion thanks to his second successive victory against Tyson Fury in December.

Usyk and Parker have 30 days to find an agreement or the WBO will order purse bid proceedings.

Britain's Daniel Dubois, who holds the IBF heavyweight belt, is already in talks to face Usyk for the undisputed heavyweight title.

Dubois was set to face Parker on 22 February but withdrew because of illness and is not looking to reschedule that fight.