A power cut at platform Skandia's London HQ meant advisers had to send client trade requests via fax on Thursday.
A major power output hit Skandia Investment Solutions' server on Thursday morning. It will be out of action until Friday.
Skandia has set up a fax number for advisers to fax through trading instructions.
"We are in the process of getting the system up and running again but it is important we do this in a controlled way," said a spokesperson. "We are doing everything we can to fix it."
In terms of refunds to customers, Skandia added it was too early to comment but said the platform would look at the issue.
"Clearly, we understand it's frustrating for our customers and we apologise for any inconvenience its caused advisers and customer."
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