OceanGate Expeditions said in a statement on Monday that it was “mobilising all options” to rescue those on the underwater vessel, which typically has five people on board.

British billionaire Hamish Harding is among the passengers, according to a social media post from a relative.

The US coast guard said on Twitter that a boat on the surface, the Polar Prince, lost contact with the submersible about an hour and 45 minutes after it began diving toward the wreckage site on Sunday morning.

US and Canadian authorities have launched a search-and-rescue operation, including aerial and surface searches, according to statements from the coast guard and Canada’s defence ministry.

OceanGate said, “We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to re-establish contact with the submersible.”

Harding’s stepson wrote on Facebook that Harding had “gone missing on submarine” and asked for “thoughts and prayers”.

The stepson subsequently removed the post, citing respect for the family’s privacy.

Harding himself had posted on Facebook a day earlier that he would be aboard the sub.

There have been no posts from him since.

The expedition headed out to sea on Friday and the first dive was set for Sunday morning, according to Harding’s post.

The expedition, which costs $US250,000 ($A365,100) per person, starts in St John’s, Newfoundland before heading out approximately 640km into the Atlantic to the wreckage site, according to OceanGate’s website.

In order to visit the wreck, passengers climb inside Titan, a five-person submersible, which takes about two hours to descend approximately 3800 metres to the Titanic.

The expedition company said that the craft has life support for its standard load of five people for 96 hours.

Harding is a holder of three Guinness World Records: longest duration at full ocean depth by a crewed vessel, longest distance traversed at full ocean depth by a crewed vessel and fastest circumnavigation via both poles by aeroplane.

Two of these feats were achieved by Harding and ocean explorer Victor Vescovo when they dived to the lowest depth of the Mariana Trench – the deepest part of the ocean – in a two-person deep-submergence vehicle in March 2021.

They spent four hours and 15 minutes traversing the deepest part of the ocean and travelled 4.6km at that depth.

In July 2019, Harding was part of a team that achieved the fastest circumnavigation of earth via both geographic poles by plane with a time of 46 hours, 40 minutes and 22 seconds.

In June last year, he travelled to space with fellow billionaire Jeff Bezos’ Blue Origin company.

The Titanic passenger ship famously sunk in 1912 on its maiden voyage after striking an iceberg, killing more than 1500 people.

The story has been immortalised in non-fiction and fiction books as well as the 1997 blockbuster movie Titanic.

with PA