A cyber attack on some of the UK’s largest banks left customers of Lloyds Banking Group unable to access their accounts.
The online assault brought down digital services at Lloyds for more than two days a fortnight ago.
The Financial Times reports the attack was carried out by an international criminal gang.
It is understood that a number of high street lenders were targeted but only Lloyds customers had trouble accessing their accounts.
A denial of service attack, where a website is swamped with traffic in an attempt to disable it, also affected Lloyds brands Halifax and Bank of Scotland, meaning many customers could not check their balances or transfer payments online.
TSB, which split from Lloyds in 2013 but shares its technology platform, was also hit.
No customers lost any money and Lloyds has declined to ‘speculate’ about the cause of the loss of service.
The National Cyber Security Centre and the National Crime Agency are currently investigating a hack into Tesco Bank, where 9,000 customers were fleeced out of £2.5m.
Other banks, including HSBC and the Royal Bank of Scotland, also suffered service outages over the past two years after their systems were breached by attacks.
Banks are frequently targeted by cyber attacks, and manage to deflect many of them.
But successful attacks reveal flaws in banks’ systems, which are often based on out-of-date technology.
Andrew Tyrie, chairman of the Treasury committee, called on regulators last year to help bolster the security of Britain’s banks after a high number of system failures.
A spokesperson from Lloyds Banking Group told the Daily Mail: ‘We had intermittent service issues with internet banking between Wednesday AM and Friday PM week before last and are sorry for any inconvenience caused.
‘We had a normal service in place for the vast majority of the period and only a small number of customers had problems accessing their accounts. In most cases if customers attempted another log in they were able to get on OK.
‘We will not speculate on the cause of these intermittent issues.’