(BANG) -Duchess Catherine has told her sister Pippa Middleton to stop acting like a ''party girl''.

The royal - who was known as Kate Middleton before her wedding - was horrified when her sister was spotted partying in New York with a string of men including hotelier Andre Balazs, 'Gossip Girl' star Chace Crawford and New York Rangers player Sean Avery and has told her to be more careful of how she acts in public.

A source told Life & Style magazine: ''Kate's mortified that Pippa's been living it up in New York and acting like a party girl. Pippa needs to remember that how she acts - her partying, her flirting with Andre and Chase - reflects badly on Kate.

''Kate has been reminding her sister not to embarrass her or the royal family. Kate's worried that Queen Elizabeth disapproves every time Pippa is out partying or making headlines.''

Duchess Catherine's husband Prince William has been trying to calm his wife down but she won't listen.

The source added: ''Prince William understands Kate's anxieties - he's been there and done that with his brother Prince Harry's antics. So he's been trying to calm her down. But Kate is very anxious about Pippa not putting a foot wrong.''