Around 315,000 people have used My Lost Account since the scheme was launched in 2008, its operators said.
And 580,000 search applications have been made, they added.
Users can find lost cash sitting in dormant accounts in banks, building societies and National Savings and Investments.
NS&I alone has reunited customers with more than £445m through its tracing service and My Lost Account, which is run by NS&I, British Bankers' Association and the Building Societies Association.
"Even small amounts of money can help with the costs of day-to-day living, so it's important people keep a track of their savings," said NS&I's retail customer director John Prout.
The BBA and BSA both had pre-existing schemes but still received 400,000 applications between them, returning £200m to customers.
The BSA's head of savings Brian Morris said: "During these tough economic times every penny counts and as such, we fully understand the importance of the scheme and will do all we can to ensure as many people as possible become aware of this free and easy to use service."
Unused cash in bank accounts that had been dormant for more than 15 years can be used to fund social investment under the the Dormant Bank and Building Society Accounts Act from 2011.
Up to £645m in forgotten funds in bank accounts has been returned over the past five years through a free tracing service run by the financial industry.