* British banks suffer string of online service problems
* Digital outages come as branches culled
* Lawmakers call on banks to explain failings (Adds comfirmation of lawmakers' inquiries)
By Emma Rumney and Lawrence White
Sept 21 (Reuters) - Royal Bank of Scotland apologised for outages that hit its online and mobile banking services on Friday, the latest in a string of such failings at British banks which have drawn criticism from customers and lawmakers.
RBS later on Friday said it had fixed the problems, which followed similar outages on Thursday that hit Barclays, the Co-operative Bank and fintech firm Cashplus.
Lawmakers on Britain's Treasury Select Committee, in letters published on Friday, called on Barclays and RBS to explain what caused the problems and set out how they would compensate customers who suffered financial losses as a result.
The outages come when Britain's lenders have been driving customers towards such services as the banks cull branch networks to cut costs. Regulators and politicians are increasing scrutiny over banks' cyber resilience.
“High street banks justify the closure of their branch networks on the basis that they are providing a seamless online and mobile phone banking service. These justifications carry little weight if their banking apps and websites cannot be relied upon," Nicky Morgan, chair of the Treasury Committee, said.
Morgan gave the banks until Friday Sept. 28 to respond to her questions about the outages.
Royal Bank of Scotland Chief Executive Ross McEwan told LBC Radio that the outage may be related to regular changes it had made to its technology, such a change to its firewall, but the cause was not clear yet.
"The team are working flat out… We feel the pain for our customers every time this happens," he said.
The RBS outage on Friday also affected NatWest, RBS's most highly rated brand among customers. The lender used Twitter to apologise to customers and said telephone banking and ATMs were working as normal.
Many customers took to social media to complain about the outage.
"So how do you expect me to pay my bills today when I work 9-5," Natalie Wilkinson said in a tweet to NatWest, asking other users to recommend a more reliable bank.
The consequences for banks of online banking problems and the subsequent scrutiny of those failings by politicians can be severe.
Earlier this year a major outage at mid-sized lender TSB, owned by Spain's Sabadell, left thousands of customers unable to access their money or make vital payments for long periods.
TSB Chief Executive Paul Pester resigned this month, following heavy criticism by the Treasury Committee and the bank's customers for his handling of the crisis.
British banks do not have long to strengthen their contingency plans for when online banking services fail.
The Bank of England and the Financial Conduct Authority in July set a deadline of Oct. 5 for British lenders to explain how they can avoid damaging IT breakdowns and respond to the growing threat of cyber attacks.
Additional reporting by Shubham Kalia in Bengaluru; Editing by Kirsten Donovan and Keith Weir