Focus: The curse of the Best Friend

Carole Caplin looked totally at home perched on her best friend's marital bed, wielding a lipliner in her manicured hand during a glossy-magazine photo-shoot inside the Blairs' bedroom. Caplin herself had chosen the £3,500 bed, a high-sprung Norwegian design that she knew Cherie and Tony would like. But while Cherie has long welcomed her bosom pal's influence on everything from the boots she wears to the bed she sleeps in, Tony could have been forgiven for fearing he was suffering The Curse of the Best Friend.

Photos taken during that shoot at 10 Downing Street showed two women revelling in an intimate girlie moment: the glamorous and self-confident friend and "lifestyle guru" bringing her much-vaunted styling skills to bear on the less assured face of Cherie Blair.

The picture spoke volumes about the dynamics in the then rock-solid friendship between a busy mother and career woman who struggles not to look like the tea lady at a village cricket match, and the former soft-porn model with a fraudster ex-boyfriend who looks part-Cheshire boutique owner, part-suburban sex kitten.

Tony Blair seems finally to have tired of the friendship, given the news that Caplin has been stripped of her Downing Street pass and can now visit her pal only by special appointment. Talk of the spurned friend "going nuclear" in reaction to this snub by publishing intimate details of life inside No 10 must have caused the Blairs sleepless nights, even on that luxurious bed. But Caplin instructed her solicitors to issue strong denials of any imminent book deal - even one that might have earned her a million pounds - and is said to have accepted the downgrading of her relationship with the Prime Minister's wife.

They will still go on seeing each other at the gym, however. While Tony has gone cold on Carole, it appears Cherie still retains some loyalty to the friend who in the past packed her suitcases, did her shopping, chose her curtains, carpets and clothes, oversaw her personal fitness training and provided nutritional, fashion and spiritual advice.

Caplin would be a hard woman for any husband to ignore - a third party knocking quietly on the door of a marriage, wanting to know if the wife can come out to play. Introduce a forceful, high-profile, demanding best friend into any marriage and there's bound to be a little trouble. Jealousy and resentment will fester, loyalties become divided, insecurities magnify. A best friend doesn't incriminate in the way a lover does, but he or she can stir the same crucifying sense of betrayal, of abandonment, of loss.

Denise Knowles, a counsellor for Relate, believes that women seek each other out for close friendship because they provide understanding, strength and emotional support. "But", she warns, "sustaining a close friendship can shrink the time you spend with your husband, so he starts to feel replaced by the friend - partly because of the time factor but also because the influence of that friendship can infiltrate areas of intimacy that the couple once had."

The model Kate Moss has attracted the attention of gossip columnists for her track record of befriending women whose relationships have gone belly-up soon after. Kate, 29, loves to party hard, and her boyfriend, publisher Jefferson Hack, seems to have no problem with that. Kate's friends are less lucky in love, however. She and Sadie Frost, 37, are bosom pals: they've been photographed strolling down London streets and soaking up the sun on beach holidays. As their friendship flourished, however, Sadie's marriage collapsed. She is now in the process of divorcing Jude Law, citing "unreasonable behaviour" that is rumoured to include an ultimatum he gave her to stop seeing Kate, whom he allegedly saw as a bad influence. Kate had planned to make Jude godfather to her daughter but announced after Sadie's split that she would never speak to him again.

Meg Mathews divorced the pop star Noel Gallagher after buddying up with Kate, following reports that Meg and Kate's rampant partying annoyed him. Kate appears to have scored a hat-trick recently with rumours that Sofia Coppola, who directed her in the video for a White Stripes single, is having marital difficulties with Spike Jonze.

Of course, it is not only gossip column favourites who have to negotiate their way through the emotional minefields of friendship and marriage. For eight years, 35-year-old Jackie Turnbull worked hard to juggle a close female friendship with marriage to a man who resented it. "My friend Sue and I met at school and we'd always been really close," Jackie said. "She was a very strong woman with opinions about everything, but lively and fun and I always enjoyed being with her."

The same did not go for Jackie's husband, John, however. He found Sue overbearing and felt she interfered in marital issues that had nothing to do with her. Jackie, from Manchester, explained: "I felt stuck in the middle of them for years but I couldn't win John round. He claimed not to like Sue but I think it came down to plain and simple jealousy. He resented the time I spent with her, and he got angry when he knew I'd confided in Sue about things that were bothering me, accusing me of divided loyalties."

Jackie was determined to hang on to both her marriage and a friendship she treasured, and had furious arguments with John as he refused to respect her relationship with Sue. In the end Jackie felt she had to choose between Sue and John, so drained had she become by the tension.

"Of course I chose my husband," she said, "but not without feeling hugely resentful that he had forced me into the loss of a friendship that meant a lot to me. I was so worn down I felt I had to take the line of least resistance in the end."

Denise Knowles says the confidences traded by close female friends can create dangerous distance in a marriage. "A wife might once have talked to her husband or asked his opinion about certain things, but she turns to her friend instead. This creates jealousy, and he puts pressure on his wife to choose between the friend and him. Men feel threatened by the intimate conversations women have with each other - 'Is she saying I'm bad in bed?', 'Is she telling her we had an argument last night?' - and become insecure and suspicious as a result."

Sharon (not her real name), 42, was given an ultimatum - "it's her or me"- by her angry husband, who couldn't bear the sight of her closest friend, believing her to be wild, irresponsible and a bad influence on his wife. Sharon, from Edinburgh, said: "That was when I knew my marriage was over ... our marriage had been dying for about three years by then, but I felt his ultimatum was just one more sneaky attempt to control me.

"My best friend is larger-than-life, a very consuming and colourful personality who's hard to ignore. That's what my husband loathed about her, but it's what I love about her. His inability to embrace - or even to tolerate - someone he knew was precious to me was the straw that broke the camel's back in my marriage, and I walked away. My best friend and I are even closer now we don't have my ex-husband's disapproving glares to contend with."

It looks as if Carole Caplin will be spending less time cosying up to Cherie on the Downing Street bedspread following the removal of the Access All Areas pass that gave her an insight into the most intimate secrets of the Blair marriage. For once in his beleaguered life, Tony might just have something to smile about. Unless, of course, Carole decides to write that book after all...