Several universities have threatened to withdraw from the UK's centralised admissions system if post-qualifications applications are introduced, casting doubt on the future of the proposed reforms.
The threat to "opt out" of the Universities and Colleges Admissions Service was made by members of the 1994 Group, which represents 19 smaller research-intensive institutions, in its response to plans for students to apply to higher education after receiving their A-level results.
Other mission groups have also voiced their opposition to the radical shake-up of admissions put forward for consultation by Ucas in October, placing the overall project in jeopardy.
Under the proposals - earmarked for introduction in 2016 - students would sit their A-level exams six weeks or a month earlier and receive their results in July rather than August.
They would then apply to just two universities and start the academic term in early October.
Universities concerned by the plans claim that the shortened window for applications would prevent them from forging vital relationships with students, especially those from poorer backgrounds who might be more hesitant to apply.
The 1994 Group's assertion that many of its institutions would have "little option but to opt out of the post-results system" represents the strongest challenge to the plans.
In its consultation response, the 1994 Group suggested that universities might alternatively supplement their admissions procedures by making "informal conditional offers to applicants" outside the Ucas system.
The Russell Group of large research-intensive universities said in its response that the proposed changes might deter students from making "aspirational" choices because they could apply to only two universities.
Modern universities from Million+ and the University Alliance also seemed opposed to the plans, saying there was too much focus on A-level students and little regard for older students or those applying for vocational courses.
Without support from universities, which fund Ucas through subscriptions, the plans are "highly unlikely to be picked up", said Matthew Andrews, chair of the Admissions Practitioners' Group of the Academic Registrars Council.
"The consultation was useful in teasing out what issues mattered, but I hope there is now a more nuanced approach to those problems that need resolving", he said.
Sir Chris Snowden, vice-chancellor of the University of Surrey, said he believed the reforms were unnecessary owing to recent changes.
"There is less rationale to move to PQA now because of improvements that have already been made to the existing system, allowing 'upgrading' for students who gain substantially better results than expected," he said.
"While to the man in the street [PQA] may seem a very logical arrangement, there are serious organisational and timing issues in respect of the whole higher and secondary education systems."
Many universities feared that they might not be able to consider contextual data such as a candidate's social background or school's performance, given the restraints of a shorter admissions process.
In its submission to Ucas, the Russell Group said "[processing] the very large volume of applications...in the short timeframe available would create pressure for assessment to...become more mechanistic and more heavily reliant on exam results".
Meanwhile, Million+, which represents post-1992 institutions, voiced concerns over a possible late-October start date, which it said would effectively reduce the first academic term to seven weeks.
And Andy Westwood, chief executive of GuildHE, which represents specialist institutions, expressed worries over "specialist application processes too, such as art and design, performing arts and teacher training, which all recruit through different admissions systems".
In its response to the consultation, Universities UK, which represents vice-chancellors, called for "consideration of alternative solutions" to the proposed Ucas model before anything was set in motion.
The shorter admissions process did not allow universities to develop a relationship with students and identify academic potential beyond A-level results, or use contextual data, it said.
The organisation's president, Eric Thomas, vice-chancellor of the University of Bristol, chaired a meeting with headteachers of secondary schools and college leaders, who expressed opposition to the plans because the school year would be curtailed, UUK added.
Despite opposition to the plans for 2016, Ucas is likely to proceed with smaller changes to processes from 2014, including removing jargon and allowing for multiple personal statements for applications.
A Ucas spokesman said that the body would publish the full findings of its consultation, along with recommendations, at the end of March.