Labour will reverse many of the coalition's changes to A-levels if it wins the next election, shadow schools minister Kevin Brennan has told England's exam regulator.
In a letter to Ofqual, Mr Brennan said Labour could not support "a policy that undermines both rigour and equity".
The current system, where AS-levels count towards full A-levels, is due to end in 2015 under government plans.
A government spokeswoman said that the changes would "enhance A-levels".
Education Secretary Michael Gove has announced that from September 2015 all A-Levels will depend on linear exams taken after two years in the sixth form.
AS-levels will still exist but as standalone exams.
Mr Brennan, writing to the chief exams regulator, Glenys Stacey, said "the weight of opposition" to decoupling the two sets of qualifications was "overwhelming".
He said the move would narrow students' A-level choices, remove a key indicator for assessing university applicants and undermine progress in widening access to higher education.
Mr Brennan's letter warns Ofqual that development work on the new exams may have to be reversed if Labour wins the election in May 2015.
"I understand that the secretary of state's position on this constitutes a policy direction to you, but in undertaking your work we think that it is important to signal clearly what our position will be following the next general election.
"It is on this basis that I write to you to inform you that a future Labour government in 2015 would not proceed with the decoupling of AS and A-levels."
The letter says that under Labour AS-levels would continue to be building blocks towards A-levels and students would continue to choose which AS-level subjects they take as full A-levels.
Mr Brennan also raises concerns about other aspects of the government's plan, including "linear assessment for all subjects at the end of two years of study, the rushed timetable for implementation, and the limited evidence base on which the proposals have been made".
A Labour spokesman added that further consultation with subject experts was needed before deciding the exact form of assessment for each A-level.
'Focus of reform'
Ministers believe linear exams will be more rigorous and help students develop deeper subject knowledge than the current modular system.
A spokeswoman for the Department for Education said that the focus of reform was to improve the quality of learning rather than to provide data for university admissions.
"We need qualifications that match the world's best and command the respect of the best universities and employers.
"For too long academics at leading universities have been concerned with current A-levels, with nearly three-quarters of lecturers having to adapt their teaching for poorly prepared students.
"Our reforms will enhance A-levels to better prepare students for higher education and ensure that competition for university places is fairer. Linear A-levels will end an overreliance on resits so all pupils develop a real understanding of a subject."
The coalition plan has drawn criticism from some university leaders. In March a group of headteachers' leaders warned that the changes risked narrowing the curriculum.
Brian Lightman, of the Association of School and College Leaders, said: "We wholly support Labour's decision to reverse a decision that was wrong in the first place.
"We hope the current government will listen to reason and abandon the plan.
"The only caveat is that if the decision does need to be amended, it must be done in a way that does not create yet more upheaval and confusion for teachers and students."
Kevin Stannard, of the Girls' Day School Trust, said he would be "very concerned if any incoming government sought to undo this reform".
He said: "Learning can become drained of its richness when time and momentum are lost in testing and retesting, as assessment becomes an end in itself rather than the means of demonstrating the value of the education being provided.
"The prospect of the reversal of a policy which has not even been implemented yet makes a mockery of long-term planning, and undermines confidence in the exams themselves."