Hearing the singer Kate Nash speak about her intensely close friendship with her best friend, Laura, 22, you might feel freaked out: “Our speaking voices have even moulded into one, and last year, we got fake married at Glastonbury,” Kate said. “There’s a part of us that just wants to hibernate together for ever.” The two write each other poems, talk to one another countless times a day and get their families together to celebrate Christmas. “My boyfriend knows I have four types of love – for my family, my friends, him and Kate,” says Laura.

But is it really so weird? Suzy Dee Holland, a 24-year-old PR, has a similar closeness to her best friend, Frances, a law student, and they often pretend to be sisters. “We’re like twins,” she says. “We say the same thing, know what the other is thinking and have a really spooky friendship. Even our time of the month is the same.” They were “born on the same day, at the same time, next to each other, and have been best friends ever since we laid eyes on each other in the incubators”.

At a time when we are increasingly starved of intimacy, such relationships can spring up as a matter of necessity rather than whim. Teens and twentysomethings – a demographic searching for stability amid changing homes, lifestyles and support networks – now have more reason than ever to turn to each other for comfort and stability. The television presenter Dawn Porter, 29, describes how she and her best friend, Louise Rigolli, 28, are “a proper little family. We’re sisterly: we could live in a box and be okay”. She moved in with Louise for six months when she was broke, sharing a bed in a curtained-off corner of a Hackney warehouse. Instead of driving each other crazy, they remember this time fondly.

There is no attraction between the pair, but in every respect other than sex, they lived like a couple, e-mailing dozens of times a day, Dawn excitedly waiting for Louise to come home, then spending the evening together having dinner and chatting. The two, who even look alike, say they “come as a couple”, are seated together at weddings and receive invitations addressed to Dawn and Louise. Last year, unknowingly, they bought each other exactly the same Christmas present and card.

These kinds of relationships are de rigueur among children, but for most of us, growing up means losing touch with our school friends, taking a lover as life partner and having a circle of friends, rather than one we prize above all others. According to MSN, the average person makes 396 friends in their lifetime, but stays in touch with 33, and considers only six to be true friends. To have a single best friend as an adult – and to use that term to describe them – is pretty unusual.

The sex and friendship expert Emily Dubberley, author of You Must Be My Best Friend . . . Because I Hate You!, explains that these intense best friendships often arise “because people have a gap in their life: for a maternal figure, a sister or even a partner. Friends provide a surrogate family, particularly now that family structures are so much looser than they were in the past”. Often, these relationships resemble a kind of schoolgirl crush that, unusually, has extended into adulthood.

“What I am, I want you to tell me,” Virginia Woolf wrote to her close friend Vita Sackville-West, who inspired her novel Orlando. Although the pair may have shared a few intimate episodes, Vita told her husband “I do love her, but not b.s.ly [backstairsly, or homosexually]. One’s love for Virginia is a very different thing: a mental thing; a spiritual thing, if you like, an intellectual thing”. The two wrote love letters, stroked each other’s hair, had a private play world in which Virginia was a weevil and Vita an emperor moth, and gave each other doggy names, “Potto” and “Towser”.

The Queens of Noize DJs, Mairead Nash, 25, and Tabitha Denholm, 33, are such intense best friends, they even married last spring. Mairead giggles: “I went a bit crazy with partying and went to Mexico to calm down. Tabitha came and met me and said she’d always support me. We went to Vegas and I thought, ‘F*** it, let’s get married and seal our friendship properly.’ This was our next phase; I needed that security at that time, it was saying, ‘We’re sticking with each other . . . we love each other.’ My boyfriend’s cool with it.” Mairead says it’s not a legally binding marriage, but the pair still wear wedding rings and call each other Mr and Mrs.

Like the love affairs that they aren’t, these relationships can also go disastrously wrong. Marianne, 32, who works in the music industry, believed she always wanted a best friend because she was an only child. After losing touch with the original “bessie” she’d had since the age of four, she found a new one, Clara. But things soon became disturbingly claustrophobic.

“It was like juggling another marriage on top of my real one,” she says. “She’d get jealous if I went for lunch with a friend and she couldn’t come. She wanted us to be the same – if I bought clothes or did something, she had to do it as well. When she broke up with her boyfriend, she pressured me to end my marriage, too.”

Initially, she tried to avoid Clara, but failed. “We had our first babies within weeks of each other and lived a few feet away. I experienced escalating panic. She was muscling in on friendships I’d made independently and wanted us to be together all the time. I felt as if I was in Single White Female.”

As in any close relationship, says Dubberley, you can also start to lose your sense of self. “If you talk to your best friend in code when you’re out with other friends, and insist on each other’s approval before you date someone, it suggests you’re becoming too entwined, rather than acting as an individual.”

Luckily, Marianne moved away with her husband’s work, but even now, she is finding it hard to shake off Clara’s devoted e-mails and calls. “I feel as if I’ve been going around interviewing my mates for the vacant position of best friend and I’m trying to stop myself. Now I’m grown up, I’m coming to realise it’s liberating to see who I want, when I want, and not be answerable to anyone.”

But for couples like Suzy and Frances, there’s no two ways about it. “I’ve always really appreciated having Frances as a best best friend,” says Suzy. “It’s so important to have someone who’s there all the time, like a surrogate boyfriend. We both know we’ll always be there to support each other with everything, and it’s nice to have that security in a relationship, whatever happens.”

Who could argue with that?

ARE YOU TOO CLOSE?

When your friend talks to someone else at a party, do you:

A Sidle over to listen in and finish her sentences for her, while coyly twiddling her hair
B Look over to check she’s not being perved on and get on with your own conversation
C Not recognise which one your friend is

When your friend needs somewhere to stay for a couple of months, do you:

A Send your boyfriend to the Holiday Inn and move her into your bed
B Offer her the sofa
C Tell her you’re sorry, but your four-bedroom house is a little cramped, what with you and the new chihuahua living there

When your friend buys the same dress as you, do you:

A Squeal with delight and suggest you coordinate wardrobes every day of the week
B Smile slightly stiffly and suggest that maybe it’s better if you don’t both wear it to your joint birthday party
C Rip up your entire wardrobe and rework your look

When your friend tells you she’s being taken on a date round the corner, do you:

A Turn up halfway through, apologise for running late, squeeze in between them, nick her fork and start eating her pasta
B Wish her luck and ask for a text update afterwards
C Tell her you’d rather not be burdened with the details of her whereabouts

When you go on holiday together, do you:

A Throw a fit when you’re given twin beds, and insist that housekeeping push them together
B Ask her not to borrow your bikini bottoms
C Lose her on day one and not see her until you bump into each other at the poolside Hawaiian luau three days later

When your friend has a child, do you:

A Check into therapy with abandonment issues
B Cross your fingers you’re the godmother and the baby is pretty enough to coo over
C Find yourself thinking, ‘So that’s where she’s been all this time – pregnant’

Mostly As: Way too close for comfort. Tongues will definitely be wagging
Mostly Bs: About as normal and adjusted as a female friendship might be
Mostly Cs: You may have some attachment issues to sort out

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