Nine people suffered life-threatening injuries in the attack
King Charles has sent his "deepest sympathy and thoughts" to those affected by a stabbing attack on a train in Huntingdon.
The King said: "My wife and I were truly appalled and shocked to hear of the dreadful knife attack that took place on board a train in Cambridgeshire last night.
"Our deepest sympathy and thoughts are with all those affected, and their loved ones.
"We are particularly grateful to the emergency services for their response to this awful incident."
The Prince and Princess of Wales also issued a message of support.
A Kensington Palace spokesman said: "Our thoughts are with the families and friends of all those impacted by the horrific events last night near Huntingdon. We also express our thoughts with the first responders and train staff who acted swiftly."
Nine people were initially thought to have suffered injuries after the stabbing in Cambridgeshire.
Four have now been discharged from hospital; however, two patients remain in a life-threatening condition.
Police confirmed the identities of the suspects arrested as a 32-year-old black British national and a 35-year-old British national of Caribbean descent.
Counter Terrorism Police are supporting the investigation, with more than 30 officers initially responding to the attack.
Superintendent Loveless added: "The train departed Doncaster at 6:25pm, and was en route to London King's Cross Station.
"Officers immediately attended Huntingdon station alongside paramedics, where armed police officers from Cambridgeshire police boarded the train and arrested two people within eight minutes of the 999 call being made. Two men were brought into police custody, where they remain this morning.
"I can tell you that these are: the first male, a 32-year-old male, a black British national and a 35-year-old man, a British national of Caribbean descent, they were both arrested on suspicion of attempted murder.
"As stated, they remain this morning in separate police stations for questioning."
Sir Keir Starmer said: "The appalling incident on a train near Huntingdon is deeply concerning".
"My thoughts are with all those affected, and my thanks go to the emergency services for their response.
"Anyone in the area should follow the advice of the police."