Ludlum was an actor and theatrical producer, who turned to writing novels when he was middle-aged.

Ludlum was published in 40 countries

His novels - more than 20 of them including The Matarese Circle and The Bourne Identity - sold in 40 countries and 32 languages.

Fans can look forward to at least three more novels, which will be published posthumously.

"It's a horrible loss for all of his fans and for his publisher," said Matthew Shear, a spokesman at Ludlow's publisher, St Martin's Press.

"Fortunately, he had been working on several books and to honour him we're going to continue to publish him."

His friend and fellow novelist Leslie Waller said Ludlum was unique and there was "nobody like him".

"There are a lot of people who know how to do a thriller, but he's one who can do a thriller about real life," Waller said.

"Almost all his books are based on reality and research. They're as close as possible to non-fiction. That's why I think they're so popular."

Fictionalised history

Ludlum often took historical characters and built his novels around them.

In his first novel in 1971, The Scarlatti Inheritance, he wrote about a group that financed Adolf Hitler's Third Reich.

Government secrets and corruption in high places were common themes in Ludlum's novels, which feature astounding twists and turns that sometimes defy belief.

Other big-selling works include The Holcroft Covenant, The Icarus Agenda, The Prometheus Deception, and The Apocalypse Watch.

The Bourne Identity, published in 1980, is being filmed, and is due for release by Universal Pictures later this year with a cast headed by Matt Damon.

Although he sold widely, Ludlum did not always win critical acclaim on a similar scale.

He was attacked for imitating the styles of fellow thriller writers John Le Carre and Tom Clancy.

"It's a lousy book. So I stayed up until 3am to finish it," wrote one reviewer in The Washington Post.