Thousands of students could have to decide on which university course to study without knowing how much they will pay for it, it is claimed.
Lawyers acting for a student planning to defer her place until 2012, when higher fees come in, say she is having to choose before fees are finalised.
They want the deadline for deferred admissions to be put back until after fees are formally set.
A government spokesman said it expected universities and Ucas to be flexible.
Last year 33,472 students deferred courses to take one or more gap years.
Universities across England have been setting their tuition fees for undergraduate courses beginning in 2012, after Parliament backed plans to allow vice-chancellors to increase them to a maximum of £9,000.
But their plans still need to approval from the Office for Fair Access and it is not due to give this until July.
This is more than two months after students will have to formally accept their offers on May 5.
After this, a formal review process means that fees may not be set finally until late summer.
On Wednesday, business secretary Vince Cable warned universities he could cut the number of student places on offer if they decided to push ahead with fees of £9,000.
Seventeen-year-old would-be sociology undergraduate Nancy Quilliam said she was in the difficult position of having to choose which offer to accept without knowing for sure what fees she would be charged.
Like many students she is planning to take a gap year, and wants to volunteer in Sierra Leone. And as she was born in August she is very young for her academic year.
She said: "It seems that there is a flaw in the government plans for tuition fees.
"At least 19,000 students will be having to choose their courses without knowing what they will be paying.
"I've got two good university options. If I knew there was a difference in the fees they are going to charge, it would definitely influence whether I choose one or another.
"But as it stands I don't know".
Ms Quilliam says the universities she has attractive offers from, Lancaster and Warwick, are likely to charge the highest maximum fee of £9,000.
But she cannot be absolutely sure because of the speed at which the changes to the university funding system are being made.
Public Law Solicitors acting for Miss Quilliam, a student at Islington Sixth Form College, in north-London, wrote to universities minister David Willetts saying: "It is extraordinary to expect people to make such a significant decision about their future and their financial liabilities without knowing the costs involved.
"Students taking up places in 2012 who are not deferring will of course not have to accept their university place until May 2012 by which time it must be assumed fee levels will have been determined.
"Ms Quilliam is being subject to a provision which discriminates against her and places her at a significant disadvantage without there being any justification or explanation for this adverse treatment."
They ask for the university admission service, Ucas, to delay the admissions deferral deadline until after fee levels have been formally set.
The lawyers add: "This is a sensible and proportionate measure which could be implemented just for this year to redress the arbitrary unfairness of a scheme clearly implemented in a hurry."
They add that "it would be unfair and irrational" not to comply with the request now the situation has been brought to the attention of the government.
A spokesman for the Department for Business, Innovation and Skills said: "The application process is managed by UCAS and the HE sector and they have already provided help to those who have announced their intention to defer.
"While the government does not interfere in the admissions process, we will look to sector and UCAS to be flexible where possible."