Arnold Schwarzenegger has undergone heart surgery and is now recovering in hospital.

The Austrian-born action movie star and former California governor "developed complications" during a scheduled procedure and needed "emergency open-heart surgery," entertainment news website TMZ. claimed.

However, in a statement spokesman Daniel Ketchell said that Schwarzenegger was in stable condition and that the operation was a success, adding that was "the most important thing".

“His first words were actually ’I’m back,” so he is in good spirits,” Mr Ketchell tweeted alongside the statement.

The operation was necessary to replace a valve that had originally been installed in 1997 for a congenital heart defect. “That 1997 replacement valve was never meant to be permanent, and has outlived its life expectancy,” Mr Ketchell said. Schwarzenegger opted for a less-invasive catheter valve replacement procedure.

An open-heart surgery team was ready during the procedure, but Mr Ketchell said their presence was not unusual in such circumstances.

After achieving fame for his roles in movies like Predator and the Terminator series, Schwarzenegger went on to win a chaotic recall election in which voters ousted then-Governor Gray Davis and chose Schwarzenegger over some 134 alternate candidates.

While he ran as a Republican, Schwarzenegger's legacy included landmark climate change legislation.

He has gone on to champion the cause and has become a vocal critic of Donald Trump, who has sought to unravel Obama-era efforts to combat climate change.

Earlier this week Schwarzenegger recorded a message supporting an organisation's fight against gerrymandering, in which officials draw congressional districts for partisan advantage.

“It is inexcusable that with Congress that has a 15 per cent approval rating and 98 per cent get re-elected. You know there is something wrong”, he said.