The friend of a British billionaire on the submersible that went missing while exploring the Titanic wreck has shared the final text he received from him.

The submersible went missing while exploring the infamous wreck, which is likely to still contain the remains of those who perished when it sank in 1912.

Passengers on the submersible had paid $250,000 (£195,000) to dive to the wreckage.

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Among them is UAE-based British billionaire Hamish Harding. Harding had previously gone to Challenger Deep, the deepest point of the ocean, as well as being among those on a Blue Origin space mission.

Retired astronaut Colonel Terry Virts has revealed the final text message that Harding sent to him.

Speaking to Good Morning Britain, he said: "We don't really talk about risks, it's known. He understood the risks for sure, there's no doubt about that.

"The last text I got was, 'Hey, we're headed out tomorrow, it looks good, the weather's been bad so they've been waiting for this.'"

"He went down to the deepest part of the ocean, set a few world records at the Mariana Trench and we talked quite a bit about the risks and the different things that they were going to be able to do. So he was very excited about it."

The trip had descended into the ocean in an OceanGate submersible. A submersible is different from a submarine as it must be launched from a larger vessel, while a submarine can leave and return to port under its own steam.

ITV

Five people are onboard the submersible, which has a limited supply of oxygen left. This means that there is a ticking clock for rescuers to locate the vehicle before its occupants suffocate.

Company OceanGate operates the deep sea tours. It had said that it is 'mobilizing all options' to rescue the vessel and its crew after it failed to surface on Monday.

Both Canadian and U.S. Coast Guards are carrying out rescue operations in an attempt to locate the submersible and its occupants before oxygen runs out.

Harding is the chair of Action Aviation group, a company which brokers the sales of aircraft including airliners and 'business jets'.

A statement from Harding on the company's website lists their clients as including 'Fortune 100 companies, international corporations, heads of state, families and VIPs from the sports, entertainment industries'.