Kensington Palace confirmed she had achieved the credential. A spokesman said: "Diving is something the duchess very much enjoys, and the Duke of Cambridge, who is of course president of the BSAC, has said in the past that he is hopeful his children will become interested in the sport."
The advanced qualification involves completing five dives - including deep and underwater navigation dives.
Those who take the course also learn how to plan dives and deal with the physiological effects and challenges of deep water.
They are taught how to explore ship wrecks, identify fish and take pictures underwater. Advanced divers need to carry equipment including a compass and a knife to be used in emergencies, such as becoming caught up in fishing nets.
The duke and duchess have admitted to a healthy rivalry, and it has often been evident during sporting challenges on official engagements.
Before the couple went head to head in a dragon boat race in Canada in 2011, the duke said: "We are both very competitive. She beats me at tennis and skiing but I am better at the rest."
William won that event but his wife got revenge last year, beating him in a race of America's Cup yachts in New Zealand.
Kelly Fisher Katz, who met the couple in Mustique while on holiday with her husband Martin, told The Sunday Telegraph: "We went diving and they went very deep, more than 90ft. Our instructor said Martin and I weren't qualified to go on the shark dives that they went on. They're quite the sportsmen."
Eugene Farrell, BSAC chairman, said: "We are thrilled that the Duchess of Cambridge shares her husband's interest in the underwater world."
- Daily Mail
