MADRID (AFP) - World free diving champion Natalia Molchanova, was feared dead Wednesday, three days after she went missing during a recreational dive in the Mediterranean near the Spanish island of Ibiza.

Spanish rescuers were still searching for the 53-year-old record-breaking Russian sportswoman and will continue to do so until Sunday, a spokesman for Spain's Guardia Civil police said.

"The search for Natalia has not officially ended but, to our great regret, the probability of success is low," the Russian Free Diving Federation said in a statement.

Molchanova failed to resurface after diving without fins to a depth of 30m-40m off the coast of Formentera island, adjacent to Ibiza, on Sunday, the international diving federation Aida and her family said in a joint statement.

It was a modest dive for Molchanova - - her record dive without the use of fins is 71m, set in May in Egypt.

But she became separated from her thee colleagues and probably got caught in a strong underwater current, the statement added.

"The cause of Natalia's disappearance is unknown, but she was doing what she loved. Natalia has a passion for free diving that burned so deep inside of her that she dedicated her life to it," it said.

"She was an inspiration to all free divers and despite being one of the fiercest competitors in the world, she was always calm and relaxed during competitions."

Spain's rescue service deployed a helicopter, boat and several divers to search for the Russian at a depth of up to 100m on Sunday.

Her family also hired an underwater robot capable of searching a radius of nearly 500m to search for her.

"We are overwhelmed and in tears here, it is difficult to find words. Why has this happened? She was such a positive and benevolent person," Andy Tutrin of the Zurich-based free diving federation Aida told AFP.

Free diving is a form of underwater diving, in which divers hold their breath instead of using a breathing apparatus such as a scuba tank.

'WORLD'S GREATEST FREE DIVER'

Molchanova, a mother of two, is the most decorated free diver in the world, with 41 world records and 23 world championship titles.

Her feats include holding her breath for nine minutes and diving to a depth of 101m using a fin.

"The world lost its greatest free diver on Sunday," New Zealand free dive champion William Trubridge, who is friends with Molchanova's son Alexey, wrote on his Facebook page.

"My thoughts are split between the memory of such an indomitable, but gentle woman, and the deepest condolence for Alexey and the family she left behind," he added.

Based at Moscow's Russian State University of Physical Education, Sport, Youth and Tourism, Molchanova abandoned competitive swimming at the age of 20 to raise her two children, Alexey and Oksana.

She discovered free diving when she was 40 and quickly developed a passion for the extreme sport, which has claimed the lives of many of its proponents.

"It's deeper in psychological terms, and more sensual than swimming in the pool. It's definitely more dangerous, but it gives extra satisfaction to go with the danger," she told the New Yorker magazine in 2009.

In 2002, French diver Audrey Mestre, 28, died while on a free dive off the coast of the Dominican Republic to 171m when her lift balloon failed to bring her back to the surface before she ran out of air.

In June 2014, leading yachtsman Laurent Bourgnon, who had French and Swiss citizenship, disappeared in French Polynesia after a going on a free dive.