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The actress, who has starred in films such as Tomb Raider, praised her fiancé Pitt last night, saying: "He's been extraordinary. He's such a wonderful man and a wonderful father and I'm very, very lucky."

Pitt, 49, said at the event held at The Empire in Leicester Square that he was "very proud" of how his partner had got through her ordeal.

"I'm just relieved that that's not going to be the thing that takes her out," he said. "The biggest fear as a father is keeping the family alive and safe and together... and she did that.

"I'm getting a bit teary-eyed thinking about it."

On Jolie's decision to go public with news of her operation, he said: "It is heroic. She realises that this is not available for everyone and it should be. The things we've gone through we've made an adventure out of it.

"As the other half, you're sitting and waiting... but I got the easy part. It's emotional.

"The importance of what people could be doing and what should be available to all overrode everything else."

Pitt was also asked about the problems that have dogged World War Z's production, including the ending having to be re-written and reshot.

"You got to stick with the film until you get it right," said Pitt, adding: "It's the best summer film I have ever made. I don't normally do this kind of affair. If you want to do it, do it big."

The movie, the first Pitt has produced and starred in, and sees him fight a zombie plague decimating the world, arrives in cinemas six months after its original launch date and over budget.

The film's studio Paramount originally gave it a budget of $150 million, but rewrites that meant 40 minutes of the movie had to be reshot saw costs spiral to more than $200 million, according to some estimates.

Paramount executive Marc Evans, who watched the director's cut in February 2012, told Vanity Fair magazine: "It was, like, wow. The ending of our movie doesn't work. I believed in that moment we needed to reshoot the movie."

Rising tensions, including rumoured clashes between the film's director Marc Forster and Pitt, saw a number of executives and crew ended up leaving the movie, the magazine reported.

The film is based on a 2006 book World War Z: An Oral History of the Zombie War, written by Max Brooks, which tells the story of a global war following a virus outbreak that turns millions of people into zombies.

World War Z goes on general release on June 21.