The Scottish Conservatives have called for major reform of the National 4 qualification.
They argue that it "should be completely reformed", so that all pupils are required to sit examinations in basic literacy and numeracy, according to the party.
Concerns over National 4 were highlighted last week by Tes Scotland. National 4 does not involve formal exams at the end of the course, relying instead on continuous assessment.
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The party's education spokeswoman, Liz Smith, said the fall in pass rates for Highers was a sign that Scottish education had "gone backwards".
"Four years ago, [first minister] Nicola Sturgeon asked us to 'judge her on education'," she said.
Fears over National 4 qualifications
"That statement now has a very hollow ring to it. No one can pretend that all is well in Scotland's schools.
"Four years on, the recent SQA [Scottish Qualifications Authority] results show that Scottish education has gone backwards.
"The pass rates for Higher and Advanced Higher are the lowest since the Curriculum for Excellence was introduced and there has been a fall in Higher pass rates for four consecutive years.
"That is a trend and a very worrying one, especially when the Higher is supposed to be the 'gold standard' of Scottish education."
Ms Smith added: "Our pupils and schools are being let down by the SNP's botched implementation of the curriculum and its qualifications system.
"Above all, National 4 should be completely reformed with new exams in basic literacy and numeracy that ensure every pupil leaves school able to read, write and count to a high level.
"A renewed focus on core skills in literacy and numeracy at the 'broad general education' phase [covering the first three years of secondary school] and National 4 level would be far more complementary to vocational training and better prepare young people for the world of work."
A Scottish government spokesman said: "Curriculum for Excellence (CfE) helps our children and young people gain the knowledge, skills and attributes needed for life in the 21st century.
"Under CfE, all teachers and other practitioners are responsible for the development of learners' literacy and numeracy, as well as their health and wellbeing, across the whole curriculum from age 3 to 18.
"This includes National 4 qualifications, which are a key part of the range of awards and qualifications on offer."
He added: "Following a review by the Curriculum and Assessment Board, which represents all parts of our education system, there remain strong educational reasons for having no exam at National 4.
"It is already being used successfully as a pathway for young people into modern apprenticeships, and college courses, including the SQA's wide range of vocational awards, national certificates and national progression awards."
This year, almost 9,000 National 4 awards – around 10 per cent of the total achieved – were given to pupils who failed the more challenging National 5 exam in a process designed to ensure that pupils who perform badly on the day of exams receive some recognition for their work.
Next year, however, the Scottish government plans to remove that safety net – the process known as “recognising positive achievement” (RPA) will end.
One secondary headteacher told Tes Scotland the risk was that students leaving school at the end of S4 next year could do so with no qualifications.