One in eight online businesses had 5% of revenues stolen by fraudsters last year, according to research from security supplier Cybersource.
More than a third of these firms lost at least 1% of revenue to fraud. The losses are the result of fraudulent online payments.
A total of 60 out of 150 businesses surveyed said they lost more to fraud in 2008 than the previous year.
The rise comes despite efforts from credit card firms and online businesses to improve customer verification,
The Verified by Visa and MasterCard SecureCode schemes have been the most popular anti-fraud investment in 2008, with 16% of merchants integrating the programmes, according to the survey.
"UK merchants continue to bear a heavy burden of fraud, including high costs and consequent profit losses," said Akif Khan, co-author of the report and the head of client and technical services for Cybersource.
"There are hopes for promising new anti-fraud developments coming from government, but even with this kind of support the merchants' principal protection will, for the most part, remain the tools and programmes that they can assemble in their defence."