A group representing UK media companies has called on the BBC Trust not to allow adverts to be published on its international websites.
The British Internet Publishers Alliance (BIPA) said the plan would hit revenue its members could make online.
Showing adverts to non-UK readers of BBC websites would also undermine the BBC's "worldwide reputation for integrity and impartiality," it added.
The BBC argues that readers outside the UK should contribute towards the costs.
While UK users pay for the website through their licence fee, international audiences are getting the service for free, the corporation says.
BIPA, which includes News International (owner of the Times, Sun and Sky), Trinity Mirror and the Guardian Media Group, said the international commercialisation of the BBC website would hurt the corporation's online rivals.
"While such revenues might seem superficially attractive as a means of augmenting the licence fee, the collateral damage to the private sector would greatly exceed the benefit," BIPA said.
The BBC Trust is due to meet next week to discuss the plans.
Key pages like the front page of the BBC News website would carry several adverts.
The income would replace BBC World Service grant-in-aid payments, which make up some of the news website's budget.
World Service would then be able to invest the money in its radio and television activities.
Any extra profit made through the advertising would flow back to the BBC to meet the government-set targets for generating commercial income.
The BBC has already approved separate plans that international users will be able to see news videos in broadband quality for the first time, with advertising clips ahead of the film paying for the extra cost to serve the content in higher quality.
BIPA said that if accepted, it was "unrealistic to believe in a future clear separation between the BBC's public and commercial output".
"Viewers and Internet users move seamlessly from one part of the BBC's empire to another," it said.
"To permit advertising on any part of the BBC's Internet world will lead to a mixed-funding model - which has failed to maintain fair competition between private and public broadcasters in other European countries where this model prevails."
The BBC Trust has replaced the corporation's governors.
Proposals have already gone through the BBC's internal journalism board and executive direction group, despite protests from hundreds of members of staff.
Visitors to bbc.com are presently redirected to the bbc.co.uk homepage.
BBC Worldwide had earlier pledged that the bbc.com site would not feature pop-up promos or the sort of ads that "give the web a bad name".
So-called geo-IP technology would ensure that only non-UK users of the site would see the advertising, although critics argue that the technology is not 100% reliable.
No responsibility can be taken for the content of external Internet sites.