Half of mobile broadband users do not know what their data limit is, while a further 24 per cent do not realise they have one, according Moneysupermarket.
A survey by the price comparison website found that three quarters of users are risking expensive charges due to the lack of awareness around usage limits.
According to the website, O2 has the highest penalty charges of all the network operators, at £200 per gigabyte over its data limit. 3 fines its customers at over £100 per gigabyte.
In stark contrast, Vodafone and Orange both charge their customers around £15 per gigabyte over their limit, while T-Mobile does not fine any customers, instead advising heavy users to switch to a more appropriate tariff.
The website said that increased usage through netbooks and laptops had increased the risk of users finding themselves lumbered with expensive penalty fines.
"Dongle users are most at risk of incurring charges from exceeding download allowances as limits on dongles tend to be much lower than fixed-line packages," said James Parker, manager of mobiles and broadband at Moneysupermarket.