BBC Online should stick to its non-commercial guns instead of eroding the brand for pocket money, according to the global analyst Forrester.

Advertising will only alienate licence payers and commercialisation will do more damage to the brand than good, said Tim Grimsditch, associate analyst of Forrester, London.

"Advertising on BBC Online would represent a major U-turn for both the website and the entire BBC content brand. In May, BBC Online began providing Yahoo! UK with content but forced the portal to remove all advertising from pages containing BBC content," he said.

Mr Grimditch's comments follow revelations this week that BBC director general Greg Dyke is considering putting ads on the BBC's Online site even though it is funded by licence payers' money.

"The BBC is one of the last advertising-free public broadcasters - advertising on BBC Online will make its huge audience question its core principles, both online and offline," he added.

Forrester thinks online advertising in the UK is already spread too thinly, particularly with just £120m of online ad revenue up for grabs this year and more than 250 sites competing for a chunk of the business.

Mr Grimsditch's research further suggests that 50% of visitors to BBC Online find banner ads irritating - 25% more than the average internet user - and they view the BBC as an advertising-free zone.

"While peeved browsers won't immediately leave for pastures new, they will slowly disappear as BBC Online becomes indistinguishable from broad-based portals like LineOne and UK Plus."