MPs have attacked the new English Baccalaureate, saying it was rushed and risks "shoe-horning" students into taking inappropriate qualifications.
Ministers should have waited until the completion of a review of the national curriculum before introducing the EBacc, the Commons Education Select Committee said in a study.
The cross-party group of MPs raised concerns at the speed at which the measure was introduced, as well as the possible impact on pupils from disadvantaged backgrounds.
The EBacc makes no allowance for the differences between pupils and could result in some being forced into taking subjects that they are not suited to, the MPs say.
The EBacc is awarded to teenagers who score at least C grades in English, maths, science, history or geography and a modern foreign language at GCSE.
Education Secretary Michael Gove introduced the EBacc at the end of last year, and the measure is now included in league tables - allowing schools to be rated on the proportion of their pupils achieving the benchmark.
The move proved controversial, as headteachers, schools and subject organisations raised concerns that they had not been told about the new measure in advance, and that subjects such as religious studies had been left out.
The committee's report says it understood the government's wish to introduce reforms quickly, but said it "regrets the launch of the EBacc before the curriculum review was completed.
It is unclear whether entering more poor pupils for EBacc subjects will "significantly" help to boost their attainment, the study says.
Concentrating on these academic subjects, could lead to schools focusing on pupils who are on the border between getting C and D grades instead of those with the lowest grades.
The report also warns: "At the same time, we believe that the EBacc's level of prescription does not adequately reflect the differences of interest or ability between individual young people, and risks the very shoe-horning of pupils into inappropriate courses about which one education minister has expressed concerns."
It is "essential" that the Government confirms how it will monitor the achievement of poor pupils in the EBacc, the committee said.
The committee's report also contains concerns that the EBacc is inappropriately named, as it is not a traditional baccalaureate.
This, the report says, could be misleading to parents, pupils and schools. It also says the government should provide more evidence that measures similar to the EBacc have been successful internationally.
Committee chairman Graham Stuart said: "There is no question that the government's motivation behind the EBacc is right in several regards.
"Of course all children should have access to a broad and balanced curriculum - including traditional, academic subjects - and of course we should be working tirelessly to narrow the gap in attainment between the richest and poorest.
"But our inquiry uncovered significant concerns about the EBacc's composition, potentially negative as well as positive impact, and the way it was introduced. We received a huge amount of evidence and the government needs to look at that very closely: indeed, if it had conducted a similar consultation, it might have avoided some of the concerns which have been expressed."
Sara Gadzik, spokesman for the Association of School and College Leaders, said: "We wholly agree with the committee, that the EBacc never should have been introduced without proper consultation or as a retrospective measure of performance.
"The EBacc will be right for some young people, and they should all have the opportunity to take the combination of subjects. But when you turn that into a performance indicator it inevitably creates a perverse incentive for schools to concentrate on those subjects at the expense of others."
Schools minister Nick Gibb said: "We believe very strongly that all children have the right to a broad and balanced education that includes English, maths, science, a language and a humanity.
"These academic subjects reflect the knowledge and skills young people need to progress to further study or to rewarding employment. It cannot be right that children from the poorest backgrounds are significantly less likely to have the opportunity to take GCSEs in these subjects than children from more advantaged areas."
Shadow education secretary Andy Burnham said: "I congratulate the committee for saying clearly and forcefully what the rest of the education world has been thinking.
"It is quite simply unforgivable to introduce a change which affects millions of young lives, without first conducting a proper process of consultation. Of course, we have been here before with this Secretary of State. When will Mr Gove learn his lesson and stop making policy on the hoof designed to appeal to headline-writers?"