Sky News understands two others on board the vessel, named Titan, are French submersible pilot Paul-Henri Nargeolet, and chief executive and founder of OceanGate Expeditions Stockton Rush.

Mr Harding's stepson Brian Szasz, said on Facebook: "Thoughts and prayers for my stepfather Hamish Harding as his Submarine has gone missing exploring Titanic. Search and rescue mission is underway."

OceanGate Expeditions, a company that deploys vessels for deep sea expeditions, said one of its submersibles had gone missing in the Atlantic Ocean and that a search is ongoing.

The US Coast Guard said the 21ft vessel has five people on board and warned that the search, taking place approximately 900 miles east off Cape Cod, has been "challenging" due to the remote location.

Rear Admiral John Mauger, commander of the First Coast Guard District, said: "It is a remote area and it is a challenge to conduct a search in that area" but added "we're deploying all available assets."

He said the search consists of both looking at the surface and subsurface. The coastguard said Titan lost contact with research vessel Polar Prince approximately one hour and 45 minutes into the vessel's dive on Sunday morning.

The main difference between a submersible and a submarine, is that the former needs a mother ship that can launch it and recover it, while a submarine has enough power to leave port and come back to port under its own power.

Read more:
What we know about the passengers on board missing Titanic submersible

OceanGate said in a statement that it was "exploring and mobilising all options" to bring the crew back safely.

The Titanic sank in 1912 after hitting an iceberg and now lies 3,800m (12,500ft) under water in the Atlantic.

Action Aviation, an aviation sales and operations company which Mr Harding is chairman of, said on Twitter on Sunday, that the billionaire was part of the Titanic expedition.

Foreign, Commonwealth and Development Office (FDCO) said: "We are in contact with the family of a British man following reports of a missing submarine off the coast of North America."

According to OceanGate, the Titan submersible is capable of diving 13,120ft "with a comfortable safety margin" and has 96 hours worth of "life support". It takes two hours to descend approximately 12,500ft to the wreck.

The vessel operates by pinging back a message every 15 minutes to signal to those ashore that it is safe, however Sky News understands that those pings have stopped.

Tourists sometimes pay tens of thousands of dollars to be taken to the wreckage of the liner. OceanGate Expeditions charges $250,000 (£195,270) per person for a place on its eight-day expedition.

A marine operations specialist outlined the difficulties in the search operation adding that "very specialised underwater vehicles" would be needed to go to the depths of where the wreck is.

Mike Welham told Sky News: "The biggest problem they've got is the depth of water that the Titanic site.

"It's about 3,800m and you need very specialised underwater vehicles to go down to that depth and they're not really readily available. So they have a major problem if they have to search and recover this vehicle."

The Titanic sank on 15 April 1912 during its maiden voyage from Southampton to New York after hitting an iceberg, with some 1,500 people dying in the disaster.