Of course men and women can be friends,’ says Anna.

‘No question,’ Eden agrees.

‘Men make the best friends,’ Jenny stresses.

‘They don’t judge,’ Alice points out.

‘Or bitch,’ Eden again.

‘Or have an agenda when you ask for clothing advice,’ I offer.

With a jarring combination of ‘yesses’, ‘exactlys’ and a single ‘so true’, all five women around the pub table celebrate this niche fact. It’s a sunny Saturday; a long, lazy girls’ lunch. But an awkward silence follows, and I think I know why.

Celia with her friend Dylan Jones, editor of the Evening Standard

‘I’m not actually sure I have many male friends.’ Jenny’s the first to come out with it. ‘Not really.’

‘I don’t have any,’ admits Eden.

‘Not one.’

The floodgates open and suddenly I’m realising that even I, who used to pride myself on having more male friends than female, don’t see them half as much as I used to.

Earlier this year I also found myself uncharacteristically flummoxed when I realised that a man I kept meeting socially and desperately wanted to be my new gay best friend wasn’t, in fact, gay – so perhaps a little too much to ask of him.

I’d already got his number by this point, still look at it sometimes, finger hovering over the ‘message’ button. But five months on, I haven’t contacted him. Why?

I’d be willing to bet that if you surveyed a whole swathe of British women, you’d find that male friends just aren’t really… a Thing. Not being a betting woman, however, I contacted the authority on friendship for some statistics: world-renowned psychologist Robin Dunbar, author of Friends: Understanding the Power of our Most Important Relationships.

He confirms that when it comes to British women’s best friends forever, ‘85 per cent are female and only 15 per cent male, but these are usually either gay men or men at the feminised end of the male distribution, ie they are good listeners and not too domineering.’

And how weird, how wrong, is that? Where are all our domineering straight male friends who never let us finish an anecdote?

In the month since that Saturday lunchtime epiphany occurred (we’ll call it a ‘pub-piphany’ because those are a Thing), I’ve conducted my own investigation into this overlooked and under-discussed cultural blind spot.

What I’ve discovered has been surprising, amusing and occasionally downright sad. Most alarming has been the initial debate over what counts as a male friend, with conversations generally following these lines.

‘Male friends? Sure, I’ve got loads. What do you mean brothers-in-law don’t count? What about my husband’s friends? The guy who sits across from me at work? He sometimes invites me to the pub with the rest of the team – if it’s someone’s birthday.’

Celia’s husband Piers Morgan

To be clear on our definitions here: a proper male friend is someone you regularly meet up with alone. He’s someone who despises the same people – real and famous – as you (nothing bonds like mutual hate) and is prone to sending you uncaptioned WhatsApp images of particularly cretinous things those people have said, done or worn.

Like good female friends, a male friend will know exactly how you take your martinis, that you like a booth in a restaurant but prefer to sit at the bar in a bar, and that you shouldn’t bother watching the latest ‘water cooler’ TV drama because it’s ‘not up your street’. So, basically, it’s husbands, minus the sex or the resentment.

Unlike female friends, they will never ask about your kids (hallelujah!) or need to get home early for their own. Owing to male blindness, they don’t tend to notice if you’ve gone through what I call an ‘ageing cluster’ (a series of increasingly inventive forms of physical deterioration), which is always a boon.

They may load dishwashers like raccoons on meth, but my god men make great friends. So why are we excluding 49.45 per cent of the UK population from our inner circles?

When I ask Professor Dunbar whether the dominance of single-sex friendships is partly down to same-sex schools and the single-sex mentality that always feels more widespread in Britain than elsewhere in Europe, he assures me the issue is ‘much deeper.

We do see it elsewhere: 75 per cent of women’s social networks consist of women, 75 per cent of men’s are male,’ he says. ‘There is a strong homophily effect (we prefer friends who are similar to us).

However, close cultural similarities (shared interests, moral or political views, etc) can override biological dissimilarities (mainly sex, personality, age and ethnicity). We see the same in social monkeys and apes.’

If they can override these things, how come the monkeys in my stratosphere aren’t doing that? Are we back to the whole When Harry Met Sally conundrum, where once even the possibility of sex is ‘out there’ the friendship is ‘ultimately doomed’ from the outset?

After attending an all-girls school until she was 18, it took Jessica Fellowes, author of The Best Friend, ‘a long time to learn how to be friends with someone of the opposite sex,’ she admits. ‘I pretty much assumed that if you liked them, then you snogged them. I did eventually have a few male friends in my 20s, but a combination of my boyfriends and their girlfriends tended to snuff these out, especially if one of us was single when the other one was coupled up.’

The theory that coupledom prohibits close friends of the opposite sex is confirmed by a number of my female friends – and I can’t help but find it depressing that beneath our forward-thinking, post-post-post-feminist façades, we seem to be stuck in a 1950s housecoat on this thing.

Two friends came out with the following sentences: ‘My husband wouldn’t like it if I said I was going out for dinner with a guy, even if it was platonic.’ ‘I stopped seeing my best male friend a few years ago. My husband kept asking why I needed a male friend when I had him.’

Full disclosure: my husband doesn’t like me having friends of the opposite sex either. Like throw pillows, I’ve had to fight for every one I possess. He’s convinced (like so many men) that any straight man is only using ‘the old friendship ruse’ to try to sleep with me.

If you are going to have male friends when you’re in any kind of established couple, everyone I’ve spoken to agrees there are ‘unspoken rules’ to be observed. A sort of boy-friend code.

If the friendship predates your relationship or marriage you’re allowed to keep it, but if you meet a man tomorrow that you’d like to have supper with, that’s pretty much out.

If you went out with the man centuries ago and both parties simply tired of one another, go on then: be friends. You’ve probably sapped any sexual tension from that relationship; it’s probably safe. If the man in question is currently dating Gisele Bündchen, that friendship is also allowed.

I don’t like these rules. They feel like social corsets – uncivilised. I think the boy-friend code needs to be rewritten so that we can all expand our friendship horizons.

Hell, there should probably even be a national ‘meet a random man and make him a friend’ day. Because aren’t we missing out on some of the most enriching relationships of our lives?

My daughter had a best male friend. Once, when she was five, I walked into the loo to find him sitting next to her on the cold, tiled floor, nattering away as she peed.

It made me sad, in the moment, that we can’t continue to have those kinds of friendships as we grow up. The ones that are so strong they override even the basic rules of etiquette, and certainly gender. Although a male bathroom-buddy? I can’t ever see my husband agreeing to that.

Celia’s latest novel, The Square, is published by Sphere, £9.99. To order a copy for £8.99 until 1 September, go to mailshop.co.uk/books or call 020 3176 2937. Free UK delivery on orders over £25.