Former Thai navy SEAL Saman Kunan was placing oxygen tanks in the cave in preparation for the rescue and died from a loss of oxygen while trying to dive back early on Friday, according to Passakorn Boonyalak, deputy governor of Chiang Rai province where the cave is located.

“His Majesty the King has instructed us to hold an honourable funeral service starting tonight,” Passakorn said.

It is unclear if the oxygen tank carried by the 37-year-old malfunctioned or simply ran out.

“We are trained to expect risks at anytime. It’s part of the job,” SEAL commander Arpakorn Yookongkaew said.

“We will never retreat. We will stay until the mission is complete,” Arpakorn added.

Lack of oxygen in the cave has proven a danger for rescue workers and the football team – a dozen boys aged between 11 and 16 and their 25-year-old coach.

The cave chamber where the group has been for the last fortnight only has 15 per cent oxygen left, according to Chalongchai Chaiyakam, an army general overseeing the rescue mission.

“Our main mission today is to lay a pipe into the chamber so the group will have more air,” he said. The pipe will have to be 4.7km long to be able to reach the chamber from the cave entrance.

“But don’t worry. We can handle it,” the general added, but did not explain how long the group can last with the current amount of air nor how long the pipe installation will take.

The group visited Tham Luang-Khun Nam Nang Non Cave, 1000km north of Bangkok near the Thai border with Myanmar, on June 23. A flash flood blocked their way out.

The discovery of the missing group by British divers late on Monday has brought relief and joy to their families and people around the world.

The group cannot be brought out of the cave immediately because they are not physically strong enough to do so and the conditions in the flooded cave remain unstable.

Despite the danger from diving, the Thai officials have confirmed it remains the main plan to get the boys out of the cave.

– AP