This isn't true unless you're in the middle of a fixed term contract. If you've come to the end of a fixed term contract or if you're on a rolling contract then the rules are different, and it's unfortunately likely that the landlord is operating within their rights. In theory the rent increase has to be 'fair' but in practice this means nothing.
Proving fair is a moving target. A friend had a similar thing and the landlord could prove that similar properties in the area went for the new price so although it was a 20% increase, it wasn't unfair and in fact, the current rent was unreasonably low compared to other places.
atz
This isn't true unless you're in the middle of a fixed term contract. If you've come to the end of a fixed term contract or if you're on a rolling contract then the rules are different, and it's unfortunately likely that the landlord is operating within their rights. In theory the rent increase has to be 'fair' but in practice this means nothing.