An employer or ex employer can say pretty much anything about you in a referance, so long as it is true. If however untruths are said in the referance, purly in attempt to harm your future employment it would be considered to be a "criminal offence" and this could carry unlimited damages, and would probably be a lot easier and cheaper to fight, than that of defamation.
Because of this, I doubt if any employer would give a false referance.
They don't have to make anything sound nice though!