The Prime Minister has said his thoughts are with families of the crew of the Cheeki Rafiki after the boat's upturned hull was found, with the life raft still in place.
Mr Cameron said: "My thoughts are with the families and friends of the crew of the Cheeki Rafiki after the sad news that its hull has been found with the life raft unused."
He added: "My sincere thanks to the US Coast Guard for leading the international search with great dedication - and to the US Navy, the Canadian authorities and to our own RAF C-130 aircraft who took part in it."
With no sign of a life raft or of debris, and with nearly 20,000 square miles having been searched, the hunt continues for another day.
Veteran yachtsman Tony Bullimore tells ITV News that there is a "strong possibility" the yachtsmen are still alive in the life raft.