British explorer is onboard a missing submersible used to take tourists to view the wreck of the Titanic.
The firm that runs the trips previously confirmed there are crew members on board the craft.
It said it was “exploring and mobilising all options” to bring them back safely.
“Our entire focus is on the crewmembers in the submersible and their families,” says OceanGate in a statement.
It says it has received “extensive assistance” from “several government agencies and deep sea companies” in its efforts to re-establish contact with the submersible.
“We are working toward the safe return of the crewmembers.”
Writing on Facebook yesterday, Hamish Harding said he was due to join the latest OceanGate expedition.
He said a dive was being attempted on Sunday, after a crew had set off from the city of St John’s, Newfoundland on Saturday.
He added the team on the vessel includes “a couple of legendary explorers, some of which have done over 30 dives to the RMS Titanic since the 1980s”.
He added that this trip was “likely to be the first and only manned mission to the Titanic” this year due to the harsh weather conditions.
Wealthy businessman Mr Harding runs a firm Action Aviation, which sells planes, and also runs its own private fleet out of Dubai.
Writing on Facebook, his stepson said: “Hamish Harding my stepfather has gone missing on submarine thoughts and prayers.”
The wreck of the Titanic
/ PAThe craft take paying passengers to see the famous wreck which sits 3,800m (12,500ft) down at the bottom of the Atlantic about 600km (370 miles) off the coast of Newfoundland, Canada.
It can seat five people which usually includes a pilot, three paying guests, and what the company calls an “content expert”.
The company charges guests $250,000 (£195,270) for its eight-day expedition to see the wreck.
A full dive from the surface to to the wreck reportedly takes eight hours in total.
The passenger liner, which was the largest ship of its time, hit an iceberg on its maiden voyage from Southampton to New York in 1912. Of the 2,200 passengers and crew onboard, more than 1,500 died.
Its tragic fate continues to have a grip on the popular imagination inspiring plays, films and books down the years.
Memorabilia connected to the ship fetches high prices and just a few months ago a plan of the Titanic used during the inquiry into the sinking of the ship sold at auction for almost £200,000.
The 32ft (9.7m) cross-section plan made £195,000 from a UK buyer when it went under the hammer.
The extraordinary plan was used by witnesses and experts to establish the cause of the tragedy, and even contains original chalk marks showing what happened.
The enormous drawing, which was mounted on linen, was so pivotal to the inquiry that it was put up in the official hearing room so witnesses could refer to it constantly.
The hand-drawn plan of the Titanic was prepared by White Star Line architects for the 1912 British inquiry into the sinking of the ship, just weeks after the disaster.
It allowed the 96 witnesses called to testify to indicate various parts of the ship using a pointer.
