The small print on some insurance and banking products is only understandable to post-graduate students, a consumer group has said.
Fairer Finance also found that a third of all insurance policy documents were written in language only accessible to those educated to university level.
It compared terms and conditions in 280 documents to a range of reading score formulae.
An estimated 16% of UK adults have a reading age of 11 or less.
The research found that no insurance policy documents could be understood by someone with an education level equivalent to an 11-year-old in the first year of secondary school.
"By communicating in a language that many people simply can't understand, banks and insurers are discriminating against large swathes of their customers," said James Daley, managing director of Fairer Finance.
"As well as being unfair, it is also bad business. If customers do not understand what they are buying, they are more likely to be disappointed. It is in everyone's interests that companies communicate clearly with their customers."
A spokesman for the Association of British Insurers (ABI) said: "All insurers want to ensure that customer documents are as clear and transparent as possible.
"They will continually review their customer materials, as they want policyholders to read policies and understand what they are covered for and what they need to be aware of. We would also encourage consumers to read what they receive from their insurers, and speak to them if anything is unclear."
A report published last year by the Money Advice Service suggested that four out of five UK adults did not read the full terms and conditions when they bought financial products, such as mortgages.
The research suggested that misunderstanding financial jargon cost UK consumers £21bn in a year - an average of £428 for every adult.