Members leafleted BBC staff as reports indicated the governing BBC Trust would rule on the proposal at a meeting on 23 May, the first to be attended by new full-time chairman Michael Lyons. It had previously been expected to give a ruling this week.
Ordered to diversify its sources of private income, BBC bosses have proposed running advertisements on the overseas-facing bbc.com website, including bbcnews.com, but a campaign emerged from opponents within BBC News Interactive - a petition has garnered more than 200 staff signatures and a letter laying out the opposition was delivered to BBC Trust members.
NUJ secretary Jeremy Dear said: "The BBC should not launch itself down this slippery slope. The public - in the UK and overseas - do not want to see the creeping commercialisation of a world-renowned public service broadcaster.
"They respect the BBC for its editorial integrity, its public service values and its ad-free space, all of which are under threat from this crass commercial adventure."
In the first indication of his opinion of the proposal, given after his appointment earlier this month, Mr Lyons said the BBC adverts plan was one of "a really complex set of issues, which do need to be weighed".