It is a standard condition of most mortgages that you must seek the permission of the lender before you rent out all or part of your property. Most lenders require a fee if you ask for permission to let.(posted 9075 days ago)If you do not seek the lender's permission to let out the property the lender can take possession action even if you are up-to-date with payments for breach of mortgage conditions. Obviously this depends whether they are alerted to the presence of tenants - most likely if you are in arrears and they send round a debt counsellor.
In the experience of many CAB money advisers, lenders are wary of giving permission to rent out properties where the borrower is already in arrears. They express concerns about the tenants' ability to pay the rent.
A few lenders are quite willing to allow tenants and will appoint receivers to collect the rent and pay it into the mortgage account.