Landlord wants to introduce lease after 3 years

I have rented a room in a house for almost 3 years. All 5 rooms in the house are currently rented on a per room basis ie we have no lease and currently would not have to cover the rent on a room if someone moved out at short notice. This hasn't happened in the 3 years i've lived here though, we have always gotten new housemates on time.

Our landlord now wants to introduce a lease. Myself & my housemates are very unhappy with this as we currently have 2 hse mates moving out and are having difficulties finding new house mates. This, i presume is why the landlord wants to bring in a lease.

Is there anything legally that can stop him from introducing one if i've lived here for that length of time? I would have to consider moving out if he did as i would not be able to afford to cover other rents. Or do i not have a leg to stand on given there is no contract or lease at the moment?

Thanks in advance.