The decline in A to D grades awarded at AS was eclipsed last week by news of yet another record-breaking A-level achievement rate.
While the AS decline is marginal, analysts believe it has led to students getting one grade in four lower than expected.
A spokesman for the Joint Council for General Qualifications denied that boards had been tougher with AS exams. He said: "Results showed that candidates were awarded the grades they earned."
But college leaders may challenge this. Kevin Conway, former principal of Greenhead College, who carries out a value-added analysis of results every year, said: "To my mind, there is no question that standards have really stiffened up."
After completing an initial analysis of 5,000 students, he told FE Focus:
"There is a definite decline in achievement. It does not mean next year's A-level grades will be worse, as students will only carry on with their best three subjects."
But many colleges predict that this will yet again hit those subjects considered hardest, such as maths, sciences and history.
Several principals said that since half the final A-level grade was dependent on the AS, they expected considerably more students to demand their right to resit modules in January.
David Linnell, principal of John Leggott College, said: "We have got fantastic A-level results but AS are lower than expected. Our AS success rate was 90 per cent. We expected 92 per cent.
"I cannot throw my arms up in the air and say they are terrible, because they still give students the basis for good results for next year. But the difference is disappointing and will leave some students with substantial ground to make-up in their A-levels."
Every year, the sixth form colleges dominated the achievement league tables, ranking alongside some the the most elite selective grammar schools.
But Dr Conway's evidence suggests lower-ability studentsin many secondary schools may struggle to make the A-level grade next year.