British Prime Minister Boris Johnson expressed his shock at author Salman Rushdie being stabbed by a man at an event in New York on Friday and condemned the attack on his freedom of expression.

"Appalled that Sir Salman Rushdie has been stabbed while exercising a right we should never cease to defend," Johnson said in a tweet.

"Right now, my thoughts are with his loved ones. We are all hoping he is okay," he said.

Former chancellor and the contender to succeed Johnson as the new prime minister, Rishi Sunak, also took to Twitter to express his shock.

"Shocked to hear of the attack on Salman Rushdie in New York. A champion of free speech and artistic freedom. He's in our thoughts tonight," Sunak said.

Foreign Secretary Liz Truss, the other finalist in the Conservative Party leadership election, said: "Disgraceful attack on Sir Salman Rushdie. People must be able to speak freely and freedom of speech must be defended.

"My thoughts are with him, his family and loved ones."

In a statement, Rushdie's publishers at Penguin Random House said: "We condemn this violent public assault, and our thoughts are with Salman and his family at this distressing time."

Rushdie, the Mumbai-born controversial author who faced Islamist death threats for years after writing "The Satanic Verses", was "still undergoing surgery", several hours after being stabbed by a 24-year-old New Jersey resident at an event in western New York State.

Hadi Matar (24) from Fairview, New Jersey has been identified as the suspect who stabbed Rushdie, the New York State Police said.