Catherine Zeta Jones reveals the secret to a successful marriage.

The 43-year-old star and her husband Michael Douglas are still enjoying a loving relationship after 13 years despite a 25-year age gap - and the actress puts it down to being respectful and giving each other the right amount of ''space''.

Asked what their secret is, she responded: ''Respect, space and a sense of humour. We spend a hell of a lot of time together, so we are careful to maintain our space.

''We recently had an extension built, to house a closet. It's like the Tardis - I go in there and never come out.

''My husband knocks on the door and says, 'Can I come in?' But I've got a TV in there, and my daughter brings her friends in - they play shop, put on all my stuff. I could be in there for the whole day, happily.''

Despite being ''so similar'', Catherine explained it's their differences which help them complement each other and maintain a healthy romance.

She told the Sunday Telegraph newspaper's Stella magazine: ''Michael is a schedule freak. He's constantly looking at the calendar and meticulously checking dates and organising weekends.

''I just wake up and say, 'What shall we do today?' I'm very spontaneous.

''Given the age difference, what's remarkable is that we are so similar in so many ways. We have a laid-back, easy-going relationship.''