The opposition of the Confederation of British Industry and the Institute of Directors could be a major blow to plans.
The institute described proposals, put forward by a government task force led by Mike Tomlinson, the former chief schools inspector, as "unnecessary" and potentially "extremely disruptive".
One of the proposed qualification's key goals is the improvement of vocational and work-based courses.
The task force set out principles of a diploma system in July. In its official response, the institute acknowledged weaknesses in the current system, which supporters of the baccalaureate say show the need for change.
It said more effort was needed to encourage young people to stay in education or training post-16 and to improve results at 16. It also claimed that A-levels had lost credibility because of rising pass rates.
Employers and universities now found it harder to distinguish between the best candidates, it added.
However, it was not clear that the bac would make the choice easier.
Improvements should be achieved within the existing qualifications structure. The institute argued against forcing students to study certain subjects post-16, as the task force proposes.
Employers were not worried about "over-specialisation" in the sixth form, a charge sometimes levelled at A-levels, the institute added.
A CBI press release said that its members were also unconvinced by the bac and were against more change.
There was slightly better news for the diploma from universities.
Universities UK, the main representative body for vice-chancellors, welcomed the "measured and long-term approach".
Universities UK praised the move to give students credit, through the diploma, for non-academic activities such as community work, sport and team-working. It also supported the drive to encourage youngsters to carry out research studies But it raised issues including ensuring that universities could get a separate idea of students' achievements in academic subjects only, and that, if youngsters were following a broader curriculum, that there was sufficient depth to their study.
Mr Tomlinson said that the majority of responses to the consultation had been positive and the message from individual employers was different from that of their representatives.
"They do not want change for change's sake, but they do say that change is necessary to meet their needs," he said.