A 17-year-old boy and the 12-year-old girl died following an incident on Wednesday afternoon. Eight other people were treated at the scene for non-life threatening injuries.

Dorset Police said an investigation had been launched and a man in his 40s, who the force said was on the water at the time, has been arrested on suspicion of manslaughter.

The force confirmed on Thursday that "early investigation indicates that there was no physical contact between a vessel and any swimmers at the time of the incident".

"We are working with partner agencies to understand what has caused the tragedy," Dorset Police said in a statement.

"We are investigating the circumstances that caused a number of swimmers to get into difficulty. Further information will be released as the investigation progresses."

Tom Saunders, a surfer who helped with the search operation, said he heard "lifeguard sirens going off, making an announcement, and then the beach got cleared either side".

"So obviously [I] knew something had gone on. We heard the helicopters coming so knew it was serious," he said.

Mr Saunders and three other surfers searched areas of the pier where the lifeguards' jetskis could not reach.

"They said there were two people missing at the time and then after about a half hour they said everyone was accounted for," he said.

Dorset Police said they received a report from paramedics at around 4.30pm on Wednesday of people requiring assistance on the beach off Bournemouth Pier.

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Ten people were recovered from the water but the 17-year-old boy and 12-year-old girl had suffered critical injuries.

Police said they were taken to hospital but died later and their families had been informed.

Detective Chief Superintendent Neil Corrigan said: "Our thoughts are with the loved ones of the young people who tragically died and we are doing all we can to support their families.

"I understand the beach was very busy at the time of the incident and I would ask anyone with information that may assist our enquiries to please come forward.

"We are at the early stages of our investigation and would ask people not to speculate about the circumstances surrounding the incident."

South Western Ambulance NHS Foundation Trust said it sent multiple teams to the incident including two air ambulances, six land ambulances and a critical care car.

The coastguard said it searched to "make sure there were no other people missing" and was "satisfied there are not".