The Reuters data-feed, which is used by firms to automatically update their own prices, went down at about 2.30pm and was restored at midnight on 12 October.
In a statement, the news and information provider said a power outage at its Docklands Technical Centre caused disruption to services, specifically its IDN-based information services in Europe.
"Customers globally may have lost access to European end-of-day data and foreign exchange transactional capabilities," it said. "Reuters is working with customers to recover any outstanding services not available."
A spokesman from Reuters would not comment on its disaster recovery arrangements.
Daniel Mayo, lead analyst in the financial services practice at analyst firm Datamonitor, said he was surprised that Reuters did not have the back-up procedures to prevent a power outage causing severe disruption to one of its main services.
"Ideally, if one datacentre goes down everything should be shifted to another site at another location," he said. "This should take about 10 minutes rather than 10 hours."
Mayo predicted that future corporate governance regulations would require financial firms to check that suppliers of key information services have adequate back-up procedures.