A-level exams in English literature have been criticised in a report by England's exam watchdog Ofqual.
A review of papers from 2005 and 2009 says formulaic questions led to some papers being less demanding.
And the report said that some exams were not challenging enough because they focused on extracts from texts rather than whole books or works.
Schools Minister Nick Gibb said the report gave "cause for concern" and would have to be addressed.
Ofqual is charged with ensuring standards in exams are maintained over time and regularly compares questions set on exam papers by the various exam boards, as well as looking at candidates' work and the grades they are given.
For GCSE English, Ofqual concludes that standards "have been maintained over time", saying that there was "no significant change" in terms of how demanding the questions were.
But the watchdog is more critical of A-level English literature.
It says there were variations between different exam boards in terms of how challenging the papers were and that "formulaic questions" meant some exams in 2009 were less demanding than in 2005.
In both years, it says a focus on extracts rather than whole books meant exams were not challenging enough, because candidates did not have to show all their knowledge of a literary work.
The report is also critical of a change in the the texts set by exam board AQA.
It says Captain Corelli's Mandolin and Catch 22 were replaced by The Adventures of Huckleberry Finn and Northern Lights and that the latter two books were not challenging enough.
Schools Minister Nick Gibb said: "Some aspects of Ofqual's reports clearly give cause for concern. We are committed to restoring confidence in GCSEs and A-levels as rigorous and valued qualifications which match the best in the world.
"The findings on English literature reflect the concerns we have already identified with the depth and scope of the reading required in key exams.
"We will be working with Ofqual and the exam boards to ensure these shortcomings are addressed as we introduce new GCSEs following the National Curriculum Review."
The government has already ordered a switch from modular-style GCSEs, with controlled assessments, to a system where exams are taken at the end of two years of study.
The exams Ofqual looked at for its report now have new specifications.
The exam board AQA said: "Overall, Ofqual found that all awarding organisations included texts of real quality, stimulus and demand and that that there had been no significant change in demand between 2005 and 2009.
"The Adventures of Huckleberry Finn and Northern Lights were texts on our legacy Literature A specification (they are not on our current specifications).
"These texts were chosen at the time because we felt they presented an appropriate level of demand for A-level students, requiring them to study connections and comparisons between texts."