Two Britons were said to be among a group of 10 tourists who had to swim more than six hours to safety after their boat sank in rough seas while travelling between islands in Indonesia, it was reported.
Two people, thought to be Indonesian, remain unaccounted for after the boat carrying 25 people went down as it was sailing between Lombok and the Komodo Islands in eastern Indonesia.
Some survivors managed to get into a lifeboat, while others had to swim three miles to a nearby island, only to find an erupting volcano, the Daily Mail said.
They had to drink their own urine while they awaited rescue and are now recovering in hospital while search teams scour 250 miles of sea between the islands as they hunt for the survivors.
A spokesman for the country's search and rescue agency said there was only one person from England on board, a 19-year-old woman called Elizabeth, but the Foreign Office said it was aware of two.
Tourists from New Zealand, Spain, France, Switzerland and the Netherlands were also among those rescued after the boat reportedly hit a reef in bad weather during the three-day journey.
A further 13 had since been rescued this morning, the spokesman for Badan SAR Nasional said, including some Indonesian crew members and an Indonesian guide.
The spokesman said: "We have rescued 23 people, including two from Holland, two from Switzerland, one from France, two from New Zealand, one from England and two from Spain. There is just one from England, called Elizabeth, who is 19 years old.
"Overall on the boat there were 25 people, 10 foreign and 15 Indonesian. Until now we are still searching for two people still in the water."
But a spokeswoman for the Foreign Office confirmed: "We are aware that two British nationals have been hospitalised following an incident in West Nusa Tenggara, Eastern Indonesia, on 17 August. We are providing consular assistance.
"We are in contact with the local authorities about the incident and stand ready to offer consular assistance to any other British nationals who may be involved."
She would not comment on the state of their injuries.
French survivor B ertrand Homassel said the boat started sinking after it was damaged in a storm, the Mail reported.
Six people were taken to safety in a lifeboat, but the others had to keep themselves afloat for more than 12 hours before they decided to swim ashore.
Speaking from a hotel in Bima on Sumbawa island, where the survivors were taken after being rescued, he said: "Six people were in the lifeboat. The others climbed on to the roof of the boat, which had not completely sunk.
"We waited until midday on Saturday. We were five kilometres (three miles) from the coast - there were many big waves separating us from the coast.
"People started to panic... Everyone took the decision to swim to the closest island, five kilometres away, where there was an erupting volcano."
Mr Homassel said the group swam for six hours before arriving at the island, Sangeang, where they survived Saturday night by drinking their own urine and eating leaves.
They were rescued today after they attracted the attention of a passing boat, the newspaper said.
Indonesian police refused to release the names of the others rescued until identities were verified, the Mail added.
Lombok is an island in the Indonesian archipelago, east of Bali. Home to around 3.1 million Indonesians, it attracts thousands of tourists every year.