Welcome to that sadly familiar episode of real-life 'Friends' - 'The One Where Your Best Mate Seems Determined To Ruin Your Life'.

Any of these sound familiar?

You find yourself unconsciously making sure that particular friend only comes round when your partner's out

Your other mates fall silent or tactfully change the subject when you mention her

You're too nervous to buy that new top/get your hair cut/change jobs until she's pronounced judgment

You lie awake at night wondering exactly what she meant when she smiled and said to you: 'Well, I think older mothers do a great job.'

You see her number come up on your mobile and instantly cancel the call - but you're not quite sure why...

If so, it's time to take action before it's too late.

'I encountered a lot of sexism in sales, which is why me and Jen - the only two women in a team of 15 - bonded against the blokes,' says Sophy, 36. 'She always looked fantastic and was happy to give me advice about my hair and clothes. But gradually, remarks like: 'Hmmm, you haven't really got Kate Moss's figure, have you?' started to sound less like friendly advice and more like - well, bitchery. It upset me but I didn't want to lose my only female work mate.'

'We'd talked about setting up our own business but then she suddenly announced that she was leaving for a rival firm. I didn't talk to her much after she left, which I put down to the pressures of her new job. Then I found out through one of my major clients that she had started up her own business - and he was placing orders with her from now on! I left her a furious message and never heard from her again. He was the first and over the next few months I lost another three. Now I realise she was using me both to make herself feel superior and to get ahead professionally. I felt so stupid.'

So whatever happened to that wonderful hippy ideal - the sisterhood? Experts say it's just another casualty of a culture focused on selfishness and success.

Alison Wolf, professor of public sector management at King's College London, says that the 'because I'm worth it generation' has led to the end of 'female altruism'. We're less inclined to be nurses or carers when we could be coining it in as professionals.Consequently, our sense of duty and 'doing good' have been replaced by a desire to succeed - whether it's at work, as a parent, as a perfect wife, or all three.

We've won the battle of the sexes, so now all that's left to conquer is each other. An undermining friend doesn't care who she tramples on to achieve her goals, or who she hurts in her quest to feel like she's number one.

So what's the best way to cope when good friends go bad?

Be honest
Yes, it's hard to admit you've been taken in. But whether she's using you to prop up her own lack of self-esteem, or is after your job or your partner, the sooner you realise it the better.

Be firm
There's no need to be rude - a casual: 'No, I don't think so,' or a polite: 'Oh, really?' will get the message across that you're no longer a pushover. Once you start asserting yourself, many undermining best friends will be off in search of easier prey.

Keep your distance
Suddenly cutting someone out of your life is rarely simple. So just make sure that you don't let your friend into any areas of your life where she could do damage. Stop sharing confidences. And if you don't like her opinions on your clothes, hair or kids, don't ask.

And finally...find someone new
False friends can ruin your life but real mates make it worth living. And there's plenty of women out there still who really will be there for you - ready with a shoulder to cry on rather than a knife in your back.