Sir Michael Caine's ''business'' relationship with his wife is the secret to their successful marriage.

The 'Interstellar' star has joked that his 42-year marriage to Shakira Caine is so great because she answers his emails.

He explained to Germany's Reader's Digest magazine: ''It's not just a partnership built on love; it's also a business partnership.

''I'll only answer the phone in an emergency. And unlike her, I wouldn't even know how to answer an email!''

And the 82-year-old star also praised his ''beautiful'' wife.

He added: ''It's also very helpful when you marry the most beautiful woman you've ever seen in your life - that's true for me even today.''

Meanwhile, Michael previously insisted he has never been ''tempted to stray'' from his wife.

He said: ''In this business, you're surrounded by beautiful women - but I've got one at home, so I've never been tempted to stray.

''We also pre-empted any of that kind of danger because I never went on location without her. Going on location is a killer: Among actors they say, 'Well, location doesn't count. I never went with that saying, because I never went on location without her. Simple.''

Michael has two children, daughter Natasha with Shakira and daughter Dominique from his previous marriage to Patricia Haines.