The NUJ and the British Internet Publishers' Alliance have both called on the BBC Trust to scrap plans for advertising on its overseas web sites.
The proposals, which are due to be discussed by the BBC Trust next Wednesday, would see advertisements placed on the corporation's international web site BBC.com.
The BIPA, whose members include major commercial online news organisations, is concerned that the BBC's plan to augment the licence fee with online advertising abroad is "the first step to more widespread commercialisation of the BBC's websites in the UK".
In a statement, the organisation said the BBC's proposal could seriously undermine the growth of British Internet content provision globally and be the first step to more widespread commercialisation of the BBC's websites in the UK."
BIPA chairman Hugo Drayton said: "The proposal by the BBC to sell advertising on its websites is a major concern to all commercial publishers who are investing in making the UK a centre of excellence in the digital age.
"As well as undermining the Corporation's worldwide reputation for integrity and impartiality, there would be a serious negative impact on all UK publishers, most of whom depend on the growing online advertising market for their revenue models. Furthermore, especially in the light of recent history, this would be a first step on an inexorable route to advertising on the BBC's publicly funded sites."
In its statement, the BIPA argued that the mixed-funding model for public broadcasters has "failed to maintain fair competition between private and public broadcasters in other European countries".
The journalists' union, meanwhile, is concerned about effect that the introduction of online adveritising would have on the BBC's journalism.
NUJ General Secretary Jeremy Dear said: "We agree that this move could lead to a greater and worrying commercialisation of the BBC website and call on the Trust to reject these damaging and flawed plans.
"The arguments against the move – technical, business and editorial – are overwhelming and the Trust should think again before opening the door to such commercialisation".
A number of BBC News staff have also campaigned against the plan, citing similar concerns.