Building society Nationwide has outsourced security services to BT to cut costs and standardise.
The telecoms giant will manage Nationwide's security monitoring services, firewalls and managed email and web access for the next five years.
This follows an outsourced agreement awarded to BT in 2008 to provide networked IT services to the building society.
Peter Stafford, IT director at Nationwide, said the system will help Nationwide to introduce standard security services and reduce costs.
The contract comes as BT announced a fall in sales for the three months to June of £0.2bn to £5bn.