A man was supposed to be on the Titan submersible but couldn't make it. Now he's helping the rescue efforts.

• A submersible with five people on route to the Titanic wreckage lost contact on Sunday.
• One man was supposed to be onboard but had to pull out. He's now assisting in the rescue efforts.

A man who said he was supposed to be onboard the submersible that went missing while traveling to the Titanic wreckage on Sunday is now helping rescue efforts.

David Concannon, an adviser to OceanGate Expeditions, the vessel's operator, told the Associated Press that he was set to be on the submersible, but had to pull out due to a work commitment.

He wrote on a Facebook page called Titanic Book Club on Monday that: "As I posted last week I was supposed to be on this expedition and, indeed, on this dive, but I had to cancel to attend to another urgent client matter."

He added: "Last night, I was called and asked to provide whatever assistance I could to ensure the safe return of everyone in the sub. Of course, I immediately agreed."

The Titan submersible had five people onboard when it lost contact with its mothership, the Canadian research vessel Polar Prince, one hour and 45 minutes into its dive in the Atlantic Ocean on Sunday, the US Coast Guard said.

It is unclear where the vessel is. Experts told the BBC that the submersible could be floating at the top of the sea, or could be on the seabed, which would make any rescue attempt extremely difficult.

With oxygen in short supply, one former submarine officer also told the BBC that passengers would have to avoid panicking to try to stop using up oxygen quickly.

Concannon, who has been giving updates about the vessel – including telling the AP on Monday that it had a 96-hour oxygen supply – asked people not to contact him as anyone who does is "interfering with more important communications that need to be made to resolve this situation as quickly and as safely as possible."