BBC's Watchdog aired concerns last night that the banks have not been honouring commitments to pay back missold PPI.
The show said: "Although the banks have committed to use all this money for repayments in reality banks seem strangely reluctant to part with it. Despite their promises to compensate, getting your money back can be remarkably difficult."
It questioned why some legitimate claims were being turned down in case studies it showed.
According to the FOS, some of the banks have been saying the customer never had PPI in the first place.
Natalie Ceeney, chief ombudsman at FOS, said: "We are getting a lot of banks saying they never had policies. When we ask more questions we found a quarter had PPI and then we go on to see if they were missold."
The programme informed of the budgets the banks had put aside to tackle the volume of claims; £550m for Santander; HSBC £1bn; RBS £1.3bn; Barclays £2bn; and Lloyds £4.3bn.
Natalie Ceeney, chief ombudsman at the FOS, said: "Unfortunately now millions of people out there have been denied justice for a long time. We are doing one of the biggest clean ups in financial services history."
The FOS reported 4000 PPI complaints calls a day with 1000 new complaints being investigated. Three quarters have been awarded compensation, it added.
According to FOS figures, for the first 6 months of this year Lloyds received 9,500 complaints and Barclays saw 19,500, with 98% and 93% upheld respectively.
Lloyds said: "The FOS figures showing 98% relate to historic processes and procedures. For a short period last year there were problems in distributing PPI compensation. This has been resolved but may have affected the FOS figures."
Complaints to the FOS are also at their lowest for three years.
