Proposals to allow students to apply for degree courses after getting their A-level results have been ditched amid concerns from schools and universities.
Ucas said there were "insurmountable difficulties" with the scheme because of differing term times and exam dates throughout the UK.
Instead, the admissions service now plans to make changes to the clearing system, the annual process which matches students without places to courses that still have vacancies.
It said there will also be improvements to the online application form, and defined offer deadlines.
Under proposals published for consultation by Ucas last October, universities would no longer have made offers to students based on their predicted grades.
The plans, seen as the biggest shake-up of the system for 50 years, would have seen students sitting their A-levels slightly earlier and then applying to university over the summer, after receiving their results.
But today, Ucas said it would not be recommending the introduction of a post-results system, known as PQA.
In a report setting out its recommendations following the consultation, Ucas said that while it had generally been agreed in principle that PQA was a "logical and desirable goal", there were "well-articulated concerns from schools, colleges and the higher education sector about the practicalities of implementation and the practical risks such a system could hold for significant groups of applicants."
The report said that one of the biggest concerns was that the four UK countries have different term and exam timetables, which could cause difficulties if the timeframe for applying to university was shortened.
There were also fears that PQA would have an effect on teaching in schools and colleges, and that the plans would need "radical" changes to the academic timetable.
"It simply would not be feasible to shoe-horn a post-results model into the current academic year," the report says.
Concerns were raised that universities' efforts to help disadvantaged students could be affected, and about institutions' abilities to match students to courses and manage student numbers.
Universities were also concerned that the timeframe to deal with applications would be too short.
And there were also fears that PQA might encourage a focus on grades, rather than a student's overall achievements and potential.
Ucas chief executive Mary Curnock Cook said: "Although many respondents to our consultation felt instinctively that a post-results process should be fairer, we heard many well-articulated concerns from schools, colleges and the higher education sector about the practicalities of implementation and the potential disadvantages for significant groups of applicants.
"However, the challenge remains to secure more accountability and accuracy of predicted grades."
Ucas is now recommending "far-reaching reform" of the clearing system.
Under these proposals there will be a gap between A-level results day and the clearing process, which is set to be rebranded, so that all offers can be confirmed.
When clearing opens, applicants will have equal access to all vacancies, with universities able to assess all students before deciding who to make offers to, rather than operating on a first come first served basis.
Other reforms will allow students to add certificates and evidence of achievements to their application, as well as tailor their personal statements.
There will also be clearly set offer deadlines.
Ms Curnock Cook said: "The clearing process was originally designed as an informal route to pair unplaced applicants with unfilled places.
"Today it needs to cater for over 50,000 applicants who want access to a process which is fair and transparent. By confirming the status of all conditional and insurance offers before the final application window opens, applicants can be sure of having fair access to any remaining places.
"This change will increase applicant choice and offer an additional recruitment opportunity for higher education institutions to fill places late in the cycle."
Mark Fuller, director of communications at the 1994 Group, which represents a group of research-intensive universities, said: "There were clear problems with the post-results applications proposals that were put forward, so it's good to see that Ucas has listened to the sector and decided against implementing them.
"The current system allows for institutions to develop relationships with prospective students, helping to support them through the process. It also encourages aspirational application choices from students of non-traditional backgrounds.
"While the principles of post-results applications are sound it would be unwise to move to a system that lacks these features."
Dr Wendy Piatt, director general of the Russell Group, said: "We are pleased that Ucas has taken on board the widespread concerns about a post-results application system.
"Despite its apparent simplicity such a system would do nothing to improve access or fairness in university admissions, reducing the time for universities to conduct fair, thorough and holistic assessments of candidates."
Universities Minister David Willetts said: "The changes Ucas propose to the admissions process will greatly improve the information and support available to applicants, allowing potential students to make more informed life changing decisions about their future.
"In particular, the proposals to replace clearing with a fairer and better managed process mark a significant change to the most fraught part of the applications cycle and will be a very welcome innovation for applicants."
A separate report published by the Institute of Education suggests that teenagers from richer backgrounds are nearly three times as likely to go to university than those from poorer homes.
The study, based on the education routes of 7,860 young people born in 1989/90, found that 66% of youngsters from the wealthiest homes went on to higher education, compared to 24% from the bottom fifth of poorest homes.
The majority of the difference is down to earlier educational attainment, author Jake Anders concludes, with parents' qualifications and school attended also playing a part.
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