Kate Middleton must find out quickly what her royal role is expected to be to prevent the same problems faced by Diana, Princess of Wales, a former Palace aide warned.

Patrick Jephson, who served as the Princess's private secretary, urged the new future Queen to turn her attention to the practical side of life within the Windsors.

''If they want her to be a wife, a pretty face, to keep quiet and stay in the background - get that straight now, not in the future,'' Mr Jephson said.

''If they want here to be more active and carry on the role Diana-style - let's get that straight too.''

He added: ''She has to agree on her position especially in relation to her future husband William. She needs to know what's expected of her. Is it going to be a joint operation?

''Does she have the necessary resources in terms of people, guidance and experience?

''If you speak to people who were around when Diana arrived, nobody had a serious talk to her about what she was going to do.

''She made it up. She developed her own style and role and in the end it got out of synchronisation with the rest of the household.''

He added: ''Kate's not just going into a marriage, she's going into a business.''

Duty, not her own career ambitions, must come first for the university graduate and one-time student, Mr Jephson said.

''It's duty first. In this position, if you don't understand what your duty is, you're going to come unstuck.''

Sacking spin doctors and following the Princess Royal's approach was among the key advice Mr Jephson had for Miss Middleton.

''Take a vow of silence, then concentrate on building up your reputation for good solid work,'' the former equerry suggested.

''In the long run, that's what people will judge her by - what she does not by what her press officers say she does.

''If she gets on with the job, a kind of Princess Anne, keeping her head down, working hard and not looking for sympathy, they will love her for it.

''If they think she's hiding behind spin doctors and there's an image being presented that isn't true, she'll be in trouble.''

On how to cope with the endless ''grip and grin'' line ups during royal engagements, Mr Jephson said: ''If she has a good purpose for meeting the general public, she will feel that much more confident.

''If she thinks it's worth doing, she will find the job relatively easy.''

The media interest Kate will face will be immense, he warned.

''The press just want to eat her up. She's a circulation booster,'' Mr Jephson said.

Of her prospects for a happy life as a royal, he said: ''She is much older than Diana was. She's got more experience. She knows William better (than Diana knew Charles).''

Referring to the night before the Princess's wedding when Lady Diana Spencer was told: 'You can't back out, Your face is on the tea towels', Mr Jephson remarked that it was never too late for Kate.

''Screw the tea towels. If you've got serious reservations, don't go through with it.

''It's such a public thing. If they don't get this one right, what's going to happen to the whole institution in the long run?''

He added: ''There will be a tidal wave of sentimental slush, but I believe what I'm saying. You've got be practical. If she was my sister, I'd tell her to get a good pre-nup.''

He warned: ''This is no ordinary marriage and the last decade has had these terrible divorces.''

But Mr Jephson said with the right approach, Kate's life as an HRH and eventually as Queen could be blissfully happy.

''If you get it right, it's the best job in the world. It can be fantastic. There is a tendency Prince Charles-style to see it as a drain.

''But they should set off with the firm intention of making it the happiest job in the world.''