What law protects me from somebody else opening my mail?

I have recently moved home and i am now finding that the person who moved into my old house has been opening my mail, calling the senders and giving them my new address. i advised somebody of importance of my new address so she is opening things like those credit card application forms etc which is a bit of a concern.
Answer
Depends what country you live inthough i would suspect it is illegal in most countries.they should put "return to sender" or "not known at this address" and put them back in the postbetter still get your mail diverted it doesnt cost muchhttp://www.royalmail.com/portal/rm/jump2?catid=400040&mediaid=600008