An American, who tackled a so-called Islamic State operative aboard a high-speed train, has gone into hospital, casting doubt on whether he will be able to testify in the trial.

Spencer Stone, then aged 23, was among passengers who helped subdue gunman Ayoub El Khazzani aboard the Amsterdam to Paris train on August 21 2015.

Thibault de Montbrial, Mr Stone’s lawyer, would not give details but said the former US airman would not testify as planned on Thursday afternoon.

France Train Attack Trial
Thibault de Montbrial, Spencer Stone’s lawyer, said he does not why Mr Stone is in hospital (Thibault Camus/AP)

“I know that he is hospitalised. I don’t know why. I don’t know how he is,” Mr de Montbrial said.

“The only thing I’m certain of is that he is not in a state to testify today. We are going to regroup this evening to gauge whether he can be heard tomorrow morning or afternoon.”

El Khazzani, a 31-year-old Moroccan, is on trial with three suspected accomplices. He risks life in prison if convicted of attempted terrorist murder.

El Khazzani boarded the train in Brussels armed with a Kalashnikov, nine clips with 30 rounds each, an automatic pistol and a cutter, according to investigators.

France Train Attack Trial
French-American Mark Moogalian, left, and his wife Isabelle. Mr Moogalian was saved by Spencer Stone after being shot in the attack (Thibault Camus/AP)

Once aboard the train, El Khazzani lingered in a restroom between cars and then emerged bare-chested with his weapons.

One of the witnesses scheduled to testify on Thursday afternoon is Mark Moogalian, a French-American who wrestled the Kalashnikov off El Khazzani – before being shot himself.

Mr Stone has said he was coming out of a deep sleep when the gunman appeared. He and Alek Skarlatos, then a 22-year-old US National Guardsman recently back from Afghanistan, snapped into action, tackling the gunman.

Mr Skarlatos was scheduled to testify on Friday.

Mr Stone, whose hand was injured by the cutter, is also credited with saving Mr Moogalian, whose neck was squirting blood.

Their heroics inspired Clint Eastwood’s Hollywood re-enactment The 15:17 To Paris.