Payday loan customers are being urged to "fight back" if they've been mistreated by a lender, by making a free complaint to the financial ombudsman.

Citizens Advice has launched a month-long campaign calling for payday customers "not to let predatory lenders get away with treating them unfairly".

The charity says customers should take their complaints to the free Financial Ombudsman Service.

Citizens Advice analysed 665 payday loan cases reported to it in the first six months of this year and found that in 76% of them, borrowers had grounds to go to the ombudsman.

It says one in five complaints were possible cases of fraud, where someone was being chased for a loan that they had never taken out.

More than a third of the cases involved lenders using continuous payment authorities to drain people's bank accounts of cash without warning. In 12% of cases, payday lenders pestered people with phone calls and texts rather than accept offers of payments that borrowers could afford.

The Ombudsman Service, which resolves disputes between consumers and financial bodies when a complaint has been unsuccessfully resolved by the firm involved, can order firms to put the situation right if it agrees with the customer's complaint. This means a borrower could get a refund or compensation.

The chances are high that the ombudsman will uphold a payday loan complaint. Of 160 complaints made to the ombudsman between April and June this year about payday loans, 72% were upheld.

Citizens Advice chief executive Gillian Guy says: "By making your voice heard you will expose the bad behaviour of lenders and put pressure on them to clean up their act, which could help stop similar problems happening to other people."