The independent organisation, which settles disputes between consumers and financial businesses, revealed that 16% of people across the UK said they'd had a problem with a financial product or service.
Its annual review of 2012/2013 also shows:
 The ombudsman received over 2 million enquiries and complaints from consumers - over 7,000 each working day.
 Tackled a record 508,881 new cases - an increase of 92% compared to last year.
 Payment protection insurance made up 74% of all cases, with the number of PPI cases rising to 378,699 - more than double the previous year.
 A quarter of people who use the service are now over 65 - double the proportion four years ago.
Looking at the trends and themes behind these figures, Natalie Ceeney, chief ombudsman, reflected: "We have seen a much stronger consumer voice in the last year - with people becoming more aware of their rights and less willing to put up with poor customer service.
"As levels of confidence in financial services have eroded, it is disappointing that we still haven't seen any significant improvement in complaints handling. Too many financial businesses still seem unable to sort out problems themselves, without the ombudsman having to get involved.
"With complaint numbers doubling, this has been another challenging year for the ombudsman. And if customer dissatisfaction remains at these record levels, the challenges we face are likely to continue for the foreseeable future."
Statistics from the ombudsman's annual review show:
 Four of the UK's largest banking groups accounted for 62% of all complaints received, up from 52% last year.
 One in four of the initial enquiries received went on to become a formal dispute - compared to one in five in the previous two years.
 The ombudsman's involvement resulted in compensation for consumers in 49% of cases.
 More consumers are realising they don't need to pay someone to make a complaint on their behalf - with a 12% reduction in the number of PPI complaints brought to FOS by claims managers.
 73% of people surveyed said they were aware of the Financial Ombudsman Service, and 71% said they trusted the organisation.
