Advice please for a worried mum

Please can someone advise me on the best way to help my 17 year old daughter. She has gotten very skinny in the last few months and I am really worried about her. I think she may be anorexic. She was a bridesmaid last week and I hadn't noticed untill then how skinny she had become. I could se her shoulder bones and her collar bones were very protrusive too. I noticed as well that she has developed a downy kind of hair on her arms. She hasn't eaten a meal in front of me for a long time and always gives excuses for not having dinner with us. (she is having it later/had something earler or a similar excuse. She has also become obsessive with exercise.

The last thing I want to do is make her situation worse but I'm worried sick. She is in denial and I know that untill she admits she has a problem things will get worse for her. I dread the phone call from someone to tell me that she has collapsed somewhere. I know that if she carries on the way she is that phone call will be sooner rather than later.

Please can anyone suggest a way to help her. I do try and encourage her but I do get stressed that she can't see what she is doing to herself, that I know isn't helping her.

Please help