The agency said a remotely-operated vehicle made the discovery near the wreckage of the Titanic on Thursday.

The hunt for the missing deep-sea vessel is still an “active search and rescue” mission after it lost communication on Sunday.

The vessel was about 700 kilometres south of St John’s, Newfoundland, during a voyage to the Titanic shipwreck off the coast of Canada.

Coastguard officials said they were “evaluating the information” following Thursday’s debris discovery.

A press conference will be held at the Coastguard base in Boston to “discuss the findings” at 8pm (1900 GMT).

Rear Admiral John Mauger, the first Coastguard district commander, and Captain Jamie Frederick, first Coastguard district response coordinator, will lead the press conference.

Founding member of the Board of Trustees of The Explorers Club, Hamish Harding, was on board the undersea craft, alongside UK-based businessman Shahzada Dawood, his son Suleman Dawood, and OceanGate’s chief executive and founder Stockton Rush, as well as French submersible pilot Paul-Henri Nargeolet.

The USCG said the ROV that made the discovery was from the Canadian Horizon Arctic ship – with the debris being found on the sea floor near the Titanic wreckage.

Assistance from the Royal Air Force (RAF) is due to arrive in St John’s on Thursday after it confirmed a request was received overnight for help with movement of additional commercial equipment.

Two RAF planes, a C-17 Globemaster and A400 Atlas, departed RAF Lossiemouth in north-east Scotland on Thursday.

A British submariner and equipment from a UK firm has been sent to help the search at the request of the U.S. Coastguard, Downing Street said.

Royal Navy submariner Lieutenant Commander Richard Kantharia, who was on exchange with the U.S. Navy, has been seconded to the search and rescue team.

OceanGate Expeditions estimated the oxygen supply on the 6.7 metre-long vessel would last 96 hours, giving rescuers a deadline of around midday on Thursday.

Experts said the chances of finding the sub and rescuing those inside was diminishing.

Former Royal Navy submarine captain Ryan Ramsey told the PA news agency: “The outlook is bleak, that’s the only word for it as this tragic event unfolds and almost the closing stages of where this changes from rescue to a salvage mission.”

The Titan is believed to be about 900 miles east and 400 miles south of Newfoundland.

It is not known how deep the vessel is, with the seabed being around 3,800 metres from the surface.