More than 3,500 complaints about financial issues relating to the coronavirus outbreak have been submitted to the financial ombudsman.
The service said firms "must do more" to ensure consumers and small businesses were treated fairly.
The number of complaints from small businesses, often about support loans and insurance, already outnumber the total from the last financial year.
The cases are still being investigated and some might prove to be unfounded.
However, the ombudsman said it expected many more issues to be raised by people left financially stretched by the economic fall-out from the virus.
Anyone in the UK has the right to take unresolved complaints to the ombudsman for adjudication.
While the service said many financial firms coped well in trying conditions, the list of complaints highlighted consumers' key concerns during recent months.
Chief among them were ruined holiday plans, and subsequent claims to insurers and credit card providers. Frustrations over refunds for cancelled weddings and concerts also featured.
Complaints from small businesses were often about insurance cover for interrupted trade.
Caroline Wayman, chief ombudsman and chief executive of the service, said: "Covid-19 has had a huge impact on virtually all elements of our lives, including our finances.
"Since measures to control the virus in the UK were put in place, we've been hearing from people who aren't happy with how their financial provider has treated them.
"Some financial businesses must continue to do more to ensure they are treating their customers fairly."
A spokesperson for the Association of British Insurers said: "Despite the unprecedented operational challenges, insurers have been delivering to customers during a very stressful period.
"This includes expecting to pay a record £275m in travel cancellations under travel insurance, £900m to those firms covered for Covid-19, as well as settling £2bn in motor claims during the second quarter of the year. The priority for insurers remains to ensure that claims are dealt with as quickly and efficiently as possible."