THE Prince of Wales's sense of duty to produce an heir led him to make an ill-founded ``marriage of convenience'', according to a new book by a prominent marriage counsellor.

Mrs Zelda West-Meads, a former spokeswoman for the marriage guidance charity Relate, also claims that Charles was continuing a British royal tradition when he took a mistress.

The remarks come in the book To Love Honour & Betray - Why Affairs Happen and How to Survive Them, which uses the royal marriage as an example of how relationships can founder due to different expectations.

The author writes that Charles felt obliged to choose a ``pure and virginal'' bride who would provide him with an heir and make a ``suitable future queen''.

But for Diana her husband was ``her knight in shining armour, who was going to love her and be hers for the rest of her life''.