The government faced calls on Friday to scrap national testing of 14 year-olds in England as it was announced that nearly three out of ten pupils would not receive results for English by the end of term.
Schools Secretary Ed Balls has already ordered an inquiry into "unacceptable" delays in the marking of this year's exams for 11 and 14-year-olds.
Concern has also been raised about the quality of the marking, conducted for the first time this year by ETS Europe, an educational testing company, with some schools saying pupils who had taken the exams had been marked down as absent.
ETS published Key Stage 3 results online for 14-year-olds on Friday, the last day of term for many secondary schools.
It said results were available for 93 percent of maths exams, 91 percent of science tests and 71 percent for English.
Results for 11-year-old pupils at primary schools were released earlier this week, but nearly one in five schools (17 percent) were still waiting for marks, the Qualifications and Curriculum Authority said.
ETS apologised earlier this month when it said that results for England's 9 million SATs papers would be at least a week late.
"Following this marking and administrative shambles, the government needs not merely to replace its contractor but to scrap Key Stage 3 tests altogether," said Liberal Democrat Shadow Education Secretary David Laws.
Kathleen Tattersall, chair of exams regulator Ofqual, told BBC radio her officials were monitoring the quality of results as they emerged on a daily basis.
"What we are seeing at the moment, and it is a very incomplete picture, is a worrying situation where some examples of poor marking have been identified," she said.
Marking appeals would soar, said John Dunford, general secretary of the Association of School and College Leaders.
"The government and (schools inspectorate) Ofsted use the SATs results to make judgements about whether schools will fail their inspections and heads can lose their jobs as a result.
"Results will be scrutinised this year as never before and the number of appeals is almost certain to rocket."
ETS was awarded a 156 million-pound, five-year contract to mark the SATS tests in February 2007.
Teaching unions have long opposed the tests, which are only conducted in England, but the government insists they are a vital part of its programme to improve the standard of education in schools.
