When a tenant sub-lets (to a third party i.e. a sub-tenant), he usually needs to obtain the landlord's prior written consent or licence to do so.
A business lease usually contains a term preventing the tenant sub-letting or parting with possession of the property. This absolute prohibition can be subject to sub-letting being permitted with the landlord's specific consent. For sub-letting, the landlord's greatest concern will be to ensure that the sub-tenant has no greater security that the tenant himself. The existing tenant remains directly liable under the lease for rent.