British police said on Sunday that they now considered only one of the two men they arrested as a suspect in the mass stabbing on a London-bound train.

"Detectives investigating a multiple stabbing on a train in Cambridgeshire can this evening confirm a 32-year-old man who was arrested is now being treated as the only suspect," said a statement from British Transport Police.

He remains in police custody on suspicion of attempted murder, it said, adding that he was from Peterborough, where he boarded the train.

"A 35-year-old man from London who was also arrested at the scene has been released with no further action," it said.

Police were alerted to an emergency on board a train between Doncaster, a town in northeast England, and London's King's Cross Station – a typically busy route – at around 7.40 pm local time on Saturday night.

Ten people were initially taken to the hospital, with four discharged shortly after.

One of the people wounded in the attack, a member of the rail staff who was on the train and tried to stop the attacker, remains in a life-threatening condition.

"Detectives have reviewed the CCTV from the train and it is clear his actions were nothing short of heroic and undoubtedly saved many people's lives," police said.

Five of those injured have been discharged from hospital.

"Our investigation is moving at pace and we are confident we are not looking for anyone else in connection to the incident," Deputy Chief Constable Stuart Cundy said in the statement.

Counterterrorism police had helped with the initial investigation, but police later said there was nothing to suggest that the incident was terrorism.

Work was now ongoing to establish the events leading up to the attack and the suspect's background, police said. A knife had also been recovered from the scene.

"Our investigation is moving at pace and we are confident we are not looking for anyone else in connection to the incident," Deputy Chief Constable Stuart Cundy said in a statement.

The suspect was arrested by armed police after the train made an emergency stop at Huntingdon around 80 miles north of London.

Prime Minister Keir Starmer called it an "appalling incident" which was "deeply concerning", while King Charles said he was "truly appalled and shocked".

Knife crime in England and Wales has risen 87 percent over the past decade, with 54,587 offences last year alone, a 2 percent rise from 2023 and among the highest rates in Europe, according to figures from Britain's interior ministry.

Interior Minister Shabana Mahmood said she was "deeply saddened", while urging people to avoid speculation about the incident. (Agencies)