Subletting is where a tenant sublets the property or any part thereof to a sub-tenant. This, however, may be prohibited by a term in the tenancy agreement between the tenant and the landlord ("the original tenancy agreement"). A tenant should therefore ascertain from the original tenancy agreement whether it contains any prohibition against subletting before entering into any sub-tenancy agreement with a sub-tenant.  Where the original tenancy agreement prohibits subletting, the tenant has to obtain the written consent of the landlord before he/she can sublet the property.  Further, the terms of the sub-tenancy agreement will usually be subject to the terms of the original tenancy agreement. The term of the sub-tenancy agreement cannot exceed the term of the original tenancy agreement. 

Source : The Estate Agents Authonty