At least 13 Britons were among those killed when a pleasure boat capsized off Bahrain in the Gulf.
The Bahrain government said 25 British nationals were among the 150 people on board the two-storey Arabic dhow.
At least 57 people are thought to have died and 67 were rescued. Bahraini Officials say it is too early to say why the boat overturned.
A UK Foreign Office spokesman warned there may be further British casualties among the missing.
The crowded pleasure boat capsized as it was returning to the port of Al Muharraq.
Bahrain interior ministry spokesman Colonel Tarek al-Hassan said that, as well as the dead Britons, there was also one Irish victim.
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Foreign Secretary Jack Straw said the British Embassy in Bahrain was helping authorities and that a rapid deployment team had been sent from London.
Foreign Office Minister Kim Howells will visit Bahrain within the next couple of days to express the condolences of the British government in person.
Acting British ambassador to Bahrain Steve Harrison has been leading a support team at the hospital where injured survivors have been taken.
He described the mood as being "pretty sombre", adding: "We've provided support for the British nationals who are there and for a number of other friendly governments who do not have embassies in Bahrain."
Murray and Roberts, the construction company which organised a party taking place on the boat, said three British employees had been confirmed dead as well as a further seven staff.
Its remaining 15 employees on the boat, including eight Britons, had all been confirmed as safe, chief executive Brian Bruce said.
UK-based company Atkins said five members of its Bahrain-based staff as well as three partners or relations had been confirmed dead.
The boat turned over in calm seas not far from the shoreline during an evening dinner cruise.
Injured survivors were taken to the Salmaniya Medical Centre in the capital, Manama.
Raymond Austin, from Kent, was one of a number of British employees of concrete company Delmon Readymix, who disembarked before the boat set sail.
His daughter, Rebecca, 18, told BBC News he and two colleagues had decided to take the employees off the boat because the number of people aboard had made them fear for their safety.
She said it was a "topping out" trip on the completion of a four-year building project in Bahrain, and would have included 150 of the main construction specialists involved, including many expatriates, some British.
Mr Austin, 50, was "shaken up and distraught" after discovering the boat had sunk with people he had spent four years working alongside still on board, his daughter told BBC News.
She said her father told her "that a number of people onboard felt it was ill-equipped, unstable and had too many people on board as they left".
Interior Minister Sheik Rashid bin Abdulla Al Khalifa, speaking on Bahrain television, said most of the boat's passengers were employees of a Bahrain-based company.
The boat capsized at about 2145 local time (1845 GMT), near one of the bridges linking Manama with al-Muharraq island, Bahrain's coastguard chief Yussef al-Ghatim told AFP news agency.
Bahrain's coastguard service launched an immediate rescue operation, reaching the scene within minutes.
US divers and small naval craft from the US 5th Fleet also joined the rescue efforts.
Graphic based on latest information available