Children should be kept at school until they can pass a basic test in reading, writing and maths, say the Tories.
The proportion of entries getting top grades rose again
Tens of thousands of children in England, Wales and Northern Ireland are receiving their GCSE results, which show a further rise in top grades.
Shadow Education Secretary Tim Collins congratulated them but said all school-leavers should have good basic skills.
"Until children get a basic qualification in literacy and numeracy they should stay in school," he said.
"We need to address the growing concern from employers that all too many school-leavers have functional difficulties with literacy and numeracy. "
The Liberal Democrats' education spokesman, Phil Willis, wants GCSEs to be scrapped.
"Whilst GCSE results remain top priority for the government, for young people and employers they are becoming an increasing irrelevance," he said.
"What the country requires is an examination system that is fit for purpose in the 21st Century where national standards in key areas such as literacy and numeracy can be obtained when students are ready to meet them.
"The current system is doing absolutely nothing for 50% of our young people who do not achieve five A-Cs and who need a radically different curriculum.
"At the other end of the spectrum, the current system is holding back highly talented young people who should be working at levels much higher than A* GCSE."
School Standards minister David Miliband rejected calls for a compulsory basic skills test.
"I do not favour raising the school leaving age until young people
pass a test, that is not going to deliver," he told the Today programme on BBC Radio Four.
The minister rejected suggestions that GCSEs should be scrapped.
"GCSEs provide a very important progress check for young people, they also
"If you think about the seven years of secondary education from 11 to 18, I
think it would be a mistake to go right throughout that period with no progress
check of the sort that GCSE provides."
A review of the curriculum and examination system for 14 to 19 year olds is underway, led by Mike Tomlinson.
He is due to report his findings to the government in a few weeks' time.
His plans include pupils studying for an over-arching diploma at various levels, with maths a compulsory element.
Tim Collins' comments echo the concerns of some sectors of industry.
The Confederation of British Industry (CBI) said the education system was not ensuring that pupils achieved acceptable standards of literacy and numeracy.
The organisation's director-general Digby Jones said: "How can school-leavers hope to succeed in the modern world of work if they cannot read and write?
"The government must focus its effort on low achievers, particularly in literacy and numeracy."
Chris Keates, acting general secretary of the teachers' union the NASUWT said: "The GCSE results show an increase in students gaining the top grades A*-A and a rise in those securing grade C and above, once again reflecting the hard work of pupils and the commitment and dedication of teachers.
"Although there is no change overall in the percentage of A*-G grades the results demonstrate a sustained high level of achievement and provide no basis for the churlish nitpicking in which undoubtedly some critics will be tempted to indulge.
"I sincerely hope that, just for once, they allow the pupils and teachers to enjoy the fruits of their labour."