In a statement, police confirmed that a 35-year-old man from London, who was also arrested at the scene, has been released without charge.

“It was reported in good faith to officers responding to the incident that he was involved in the attack, and following enquiries we can confirm that he was not involved,” police said.

The 32-year-old suspect remains in custody. A knife was recovered from the scene, and authorities said he had boarded the train at Peterborough station.

Police said six people were injured in the attack. Five have been discharged from hospital, while one — a member of LNER rail staff who attempted to stop the attacker — remains in a life-threatening condition.

“Detectives have reviewed CCTV footage from the train, and it is clear his actions were nothing short of heroic and undoubtedly saved many people’s lives,” police added.

Deputy Chief Constable Stuart Cundy said the investigation was progressing rapidly.

“Our investigation is moving at pace, and we are confident we are not looking for anyone else in connection with the incident,” Cundy said. “Specialist detectives are examining the background of the suspect and the events that led up to the attack.”