The Letting Centre

I have discovered my tenants are allowing a 3rd person to live at my property and they have taken a deposit and are charging the 3rd person weekly rent. The 3rd person is not on the tenancy agreement and the tenancy agreement clearly states: Not to assign or sublet any part of the Property or let any other person live at the Property other than the persons defined as tenants and not to receive any paying guests.  The agreement also states if there is a breach of any other of the Tenant's obligations, the Landlord may re-enter the Property and this Agreement shall end.

Should I immediately end the contract or should I still provide written notice?