Students should be allowed to receive offers from universities only once they have received their final grades, the admissions body is expected to say.
Ucas is due to set out its recommendations for reforming the universities' admissions system this week.
John Cope, director of strategy, policy and public affairs at Ucas, said the organisation will be "cautiously backing" a model whereby offers would be based on students' actual grades rather than predicted grades.
But he said the other option on the table – where students would apply to university and receive offers after A-level results day and then start their degree courses in January – was a "step too far".
Earlier this year, the Department for Education launched a consultation on moving to a post-qualifications admissions (PQA) system in England amid concerns about the accuracy of predicted grades.
During an online event on the government's consultation on reforming the admissions system, Mr Cope said Ucas would be supporting a move to post-qualification offers – where students apply in the usual way during term-time, but offers would only be made after results day in the summer.
But he said the government would have to ensure that schools and colleges have the right resources and support in place during the summer period to help students with deciding which offers to take up.
Mr Cope said: "It would require the government to think very carefully around how do you make sure that careers advisers and teachers are around in the August period to make sure that when students are making those decisions about university they make it in an informed way."
One option being considered by the government would see students apply to university and receive offers from institutions after A-level results day – and the start of university could be pushed back.
Speaking at the online event, which was hosted by exam board AQA, Mr Cope said Ucas had "ruled out" this option as it was "not practical".
He said: "The idea of moving the academic year to January is a step too far for us. It would put us out of sync internationally so it would make it very difficult in the international student market."
Mr Cope said Ucas was due to publish a more detailed report on their preferences for admissions reform on Friday.
"We will be cautiously backing post-qualification offers," he said.
Mr Cope highlighted that PQA has been called for repeatedly over the years due to concerns about the accuracy of predicted grades, as well as a lack of transparency around entry requirements.
He added: "There are certainly issues around predicted grades and moving away from them is something that Ucas would welcome."
But Mr Cope said he did not think moving to a new system would be able to be implemented before 2024.
The government's consultation on admissions – which is open until 13 May – was launched after vice-chancellors and Ucas outlined proposals in November last year to move away from predicted grades.
AQA is supporting a move to a PQA system – whereby students would receive university offers based on actual results rather than predicted grades.