Two British divers who helped rescue the Thai youth football team from a flooded cave had earlier saved four adults also stranded in the complex, according to a new report on the mission that gripped the world in July.

The 12 boys and their coach from the Wild Boars football club were trapped on June 23 by rising waters as they explored the zigzagging passages of the Tham Luang cave in northern Thailand. Officials leading the rescue effort called on the expertise of the world's small caving community, many from Britain, who formed a crack diving team with Thai Navy Seals.

On the ninth day of the rescue, British divers Rick Stanton and John Volanthen found the youth group dishevelled and thin on a muddy bank, where they had survived on rainwater dripping from rocks. They were pulled out a week later in a three-day extraction, during which the boys were sedated and carried to the cave entrance.

But days earlier, it emerged, the Britons had unexpectedly found four Thai adults who had themselves become trapped by floodwater while trying to find the children.

The Associated Press reported that the four were members of the Thai Well Water Association. The association's president, Surapin Chaichompoo, on Wednesday recounted how he and three employees spent two days draining water from the cave in an attempt to help the rescue. But after pausing to take a nap, they found themselves trapped inside with the water rising.

Rescue personnel work at the Tham Luang cave complex in the northern province of Chiang Rai, Thailand July 4, 2018. (Thai Navy Seal/Handout via REUTERS)

He said that on their third day, two British divers searching for the boys stumbled upon them and improvised a rescue. Mr Surapin said he was unable to contact them afterward, but would still like to thank them.

Droves of well-meaning volunteers and local residents had poured into the area near the border with Laos and Myanmar to try to help.

Speaking this week at a "Hidden Earth" cave-community event in the British county of Somerset, Stanton said he and Volanthen had accidentally come across the four Thai water company workers on June 28. They had been trapped for around 24 hours and their situation was desperate, according to the caving news website Darkness Below in an article summarising Stanton's presentation.

Muddy water was "churning, swirling and eddying around them", it said, and the water level was rising. But Stanton and Volanthen had only their personal breathing equipment with them. They solved the problem through a staggered approach.

First the two Britons dived back through the flooded passageway to reach the next level of dry ground en route to the cave entrance. One of them took off his gear and the other swam back to the Thais with the breathing equipment to bring them out one at a time.

This exposed the waiting diver to possible drowning had the area flooded suddenly. But despite a brief technical glitch with the gear, all made it out safely, the article said.

The scenario foreshadowed concerns about the later rescue of the boys, who were not experienced divers, prompting fears they could panic.

"It's one of the most remarkable aspects of this whole event that such an amazing thing was a mere sideshow that never even made the press," Les Williams, chairman of the British Caving Association, was quoted as saying.

Details of the incident are still captivating the public, prompting books and film treatments in a scramble to cash in on the saga.

Interest has been further fuelled by a lawsuit against tech billionaire Elon Musk brought by a different British caver who lives in Thailand.

Musk has called the caver, who was integral to the rescue because of his knowledge of Tham Luang, a "pedo" on his Twitter account. He gave no evidence to support the allegation.