A broadband comparison site has called for UK mobile broadband providers to come clean about average download speeds, after research indicated that they were delivering less than a quarter of the performance promised.

Broadband-expert.co.uk has revealed that the download speeds experienced by most consumers are a mere 24 per cent of those advertised. Of the 3,342 connections it tested between 1 March and 31 August 2009, the average download speed was only 1.1Mbit/s compared with a pledged maximum of 4.5Mbit/s.

“It is completely unreasonable for a provider to advertise unrealistically high speeds that the vast majority of customers will never receive," said Rob Webber, broadband-expert’s commercial director.

"Advertising in this way will not help the long-term growth of mobile broadband or the reputation of the providers if customers feel they are being misled."

While Vodafone recorded the fastest actual speed at an average of 1.3Mbit/s, it delivered the lowest percentage of advertised speeds – just 18 per cent of its promised 7.2Mbit/s. T-Mobile delivered the slowest actual speed at 0.9Mbit/s or 20 per cent of advertised speeds, while 3 offered the highest percentage of advertised speeds. It clocked in at 1.2Mbit/s or 33.3 per cent of pledged performance.

In comparison, broadband-expert also tested 94,546 home broadband connections and found the average speed was 3.6Mbit/s or 44 per cent of advertised maximum speed.

These findings led Webber to suggest that consumers might be advised to view mobile broadband as a complement to rather than a replacement for home broadband.

“Continued developments in the mobile broadband industry will undoubtedly allow it to become a true competitor in the future, but at the moment – as our research shows – there is still a way to go,” he said.

But research elsewhere has also revealed that customers need to be wary of the hidden charges written into the small print of both their home and mobile broadband contracts as they are netting providers a hefty £370m in additional revenues per annum.

Opinium Research, which undertook the survey among 2,000 online consumers on behalf of moneysupermarket.com, found that one in six respondents was paying an average of £44 per year in extra charges. The figure rose to £61 among customers in their twenties.

Almost half of those that incurred such charges had no idea they were written into their contracts. The fees related to everything from paper billing to non-direct debit payments and exceeding download limits and are perceived as a means for providers to claw back revenue as call costs continue to fall.