I have a 4 year old Bearded Collie who has a lovely temperament with people.  When I am out walking her off the lead, she ignores other dogs however, when I have her on the lead and another dog approaches her, she becomes very aggressive and has even nipped a labrador puppy outside school.  Has anybody had a similar problem?  Any ideas I could try before I take her to the vet (to be referred to a behaviourist not to be put down!!)

I have this with my staffie, i just think he hates being on the lead. I have to say that its in the park where I get most of the trouble and seeing that he is so good with people and other dogs, i leave him off the lead. The only time i put him on is if i see another dog with a ball, walk past them and let him off again.
Good luck

Some dogs tend to be less comfortable around others when they are on the lead as they do not have the option of flight.  Also some owners inadvertently train their dogs to be aggressive on the lead because they tighten it in the presence of other dogs thereby giving their dogs a signal that there is something to be worried about.

The other thing to remember is that dogs who invade another's personal space are very rude and so are the owners who allow this to happen...............