Vittorio Missoni's relatives are stepping up their search for the missing Italian fashion mogul by using a submarine to look for his private plane underwater.

The small aircraft transporting the 58-year-old co-owner of the famed Missoni label, as well as his partner and four others, disappeared off the coast of Venezuela on 4 January (13).

His family is refusing to give up hope that Missoni will be found alive, but they have decided to use a deep-sea research vessel to hunt for plane wreckage.

In a statement, the designer's brother Luca Missoni says, "(We) continue to hope for the safe return of Vittorio. I will personally return to Caracas in February to oversee the continuing underwater search being carried out by submarine Deep Sea, a specially equipped vessel for deep water searches."

The fashion executive is thought to have been returning from a holiday with his partner Maurizia Castiglioni and two of their friends when the plane vanished. Two pilots were also onboard the aircraft when it disappeared.