Some 78 per cent of mobile broadband users risk facing charges for unknowingly exceeding their download limit, new research shows.
According to a survey by moneysupermarket.com, T-Mobile was the only provider not to charge dongle users for exceeding their mobile broadband plans.
While the operator was happy to advise customers to switch to more suitable packages, others were not so scrupulous.
Orange, Virgin and Vodafone were all found to charge similar amounts for every GB of data downloaded out of bundle - £14.64, £14.95 and £15 respectively.
However, 3 and O2 were much harsher with their penalty fees; the former charges £102.40 per GB with a disconnection cap at £50, but the latter demands £200.70 per GB and does not implement any sort of cap, the website warned.
James Parker, Manager of Mobiles and Broadband at moneysupermarket.com, said: "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.
"Whilst the portability is great, users need to be aware that a dongles are most suitable for light users, fixed-line is a better value option for heavy users."
Worryingly given the fees attached to downloading too much, 24 per cent of people admitted to having no idea if they have a limit in place on their mobile broadband internet service.
More than half (54 per cent) of those who did know were unsure of what their limit actually was.
Recently, the EU introduced a new cap on roaming charges for internet usage, limiting the wholesale cost of mobile broadband to 1 (85p) per Mb.