They are believed to have been swept way by the current more than 24 hours ago. There has been no sign of them despite an intensive search.
Ms Mitchinson and her husband Ernest run a dive company in Bali called Reefseekers, and she is described on diving websites as experienced and knowledgeable.
Ernest Lewandowski told the AFP news agency the pair had been leading two separate groups of tourists on the dive.
He noticed his wife's group was missing when his group surfaced an hour after they entered the water.
He said the search was being hampered by a lack of fuel for aircraft.
Friday, June 06, 2008
British scuba divers missing after Komodo National Park dive
Five scuba-divers are currently missing after a dive in the waters of the Komodo National Park. Three of them are British and have been named as Kathleen Mitchinson, Charlotte Allin and James Manning. Our prayers are with them and their families during this time. BBC News has the full story.