In its response to a consultation by the Universities and Colleges Admissions Service on post-qualifications applications (PQA), Supporting Professionalism in Admissions said nearly all types of students would be negatively affected by the proposed overhaul.
The SPA response to Ucas' consultation, which closed on 20 January, was drawn up by a steering group led by Sir Steve Smith, vice-chancellor of the University of Exeter.
Other steering-group members include the Academic Registrars Council's Admissions Practitioners Group, Universities UK and the Higher Education Academy.
With the admissions process concentrated in a relatively brief post-results window, many students would struggle to access the right information, advice and guidance, the group said.
Students from poorer families could also be deterred from making aspirational applications to elite universities if they did not have the right support within this short period, SPA said.
Institutions would have less time to consider contextual data beyond a student's exam marks and might also have to cancel summer schools for disadvantaged students, it added.
"Aspirational choices for some widening participation applicants may be negatively affected and restricted by a post-results system," SPA director Janet Graham said in the response.
"Without reliable, professional advice in relation to the interpretation of [institutions'] published entry criteria, it is highly likely these applicants could be disadvantaged and may be deterred from applying.
"Such students may not have the background knowledge or social networks that other students may have."
Losing the predicted grades system could also be detrimental to some students, Ms Graham said. "Schools and colleges have commented that an early conditional offer motivates some of their learners to improve over the final year," she added.
The SPA's response also disputes whether the proposed system would save money because savings in the admissions procedure would be offset by extra spending on outreach and marketing.
Denying that the current system was "broken", the report concludes: "The UK has one of the best central applications processes there is. If the system is to be changed radically, the benefits...must outweigh any costs."