Dozens of Britons are believed to have been safely rescued after a luxury cruise liner ran aground and later keeled over off the Italian coast.
But three people on board the ship were confirmed dead by the Italian coastguard and divers were scouring the submerged hull for around 40 people who remain missing.
The Costa Concordia, which was carrying more than 4,000 passengers, experienced trouble a few hundred metres from the tiny Tuscan holiday island of Giglio on Friday evening after apparently sailing off course.
There was panic as desperate passengers scrambled to evacuate the stricken vessel as it began to take on water and list to the right, with some opting to jump into the sea.
The Foreign Office said it was unable to verify reports that all 37 British nationals were safe after Costa Cruises announced the 25 passengers and 12 crew members had been accounted for. The three dead were said to be two French passengers and one Peruvian crew member.
The ship's captain, Master Francesco Schettino, and the first officer were being questioned by prosecutors, Italian state TV reported. Some unconfirmed reports indicated they were being held on suspicion of "abandoning ship" and "multiple manslaughter".
The first sign that something was wrong on the vessel was a power black-out and large 'boom' noise at around 10.30pm on Friday, according to passengers.
The ship then ran aground off Giglio and a massive coastguard rescue operation involving helicopters and rescue boats was immediately launched. As the sun rose on Saturday, the ship could be seen almost completely on its side.
Gianni Onorato, the President of Costa Cruises, said in a statement on the company website: "I want to express our deep sorrow for this terrible tragedy that devastates us."
The Foreign Office gave contact details for the British Embassy in Rome for concerned friends and relatives. The telephone number is (+39) 06 4220 0001, and the Foreign Office in London can be contacted on 0207 008 1500.