LCH.Clearnet suffered a serious IT failure on 31 December, affecting payment processing and hindering the clearing house's contingency plans, according to the Bank of England.
The revelation comes in the BofE's payment systems oversight report for 2012, which says that the "major" failure "affected operational processes including payment arrangements."
It continues: "The nature of the problem created obstacles to reverting to contingency arrangements and also hindered internal and external communication."
Systems were restored by the early evening and payments settled as normal, says the BofE, while LCH.Clearnet is investigating the causes and has developed plans to prevent a recurrence.
This week the clearer's shareholders will vote on a deal that will see them offload a controlling stake to the London Stock Exchange.
The BofE report also notes an outage at financial messaging outfit Swift in July but says that the timing meant that there was no significant disruption to the UK payments system.