GCSE results could fall this year amid major upheaval in the exams system, it has been predicted.
Attempts to "secure standards" along with changes to key GCSEs and moves by students to sit different exams could combine to cause a drop in the pass rate, according to one education expert.
The prediction comes the day before teenagers across England, Wales and Northern Ireland are due to receive their results.
Professor Alan Smithers, director of the Centre for Education and Employment Research at Buckingham University, said it was "likely" that results will drop this year.
Last summer, the proportion of GCSEs awarded at least a C fell for the first time in the exam's history, with 69.4% getting this grade or higher, down 0.4 percentage points on 2011.
There was also a fall in the proportion of GCSEs awarded the top grades and drops in the percentage of English, maths and science GCSE entries achieving passes at A*-C.
Prof Smithers said: "I think it's likely that it will drop. Last year it came down a bit at all levels.
"What has happened is the pass rate has gone up year on year because everyone had a vested interest in that happening."
But he added that there is now a "true regulator" - Ofqual, which has said that standards should be comparable year on year.
Prof Smithers said that the introduction of the Government's new English Baccalaureate, which is awarded to students who gain at least a C at GCSE in English, maths, science, history or geography and a language could also have an effect.
"There are a number of people who might otherwise not be taking history or geography and a foreign language (who) may well be taking them as their school attempts to get the EBacc."
These students may therefore not be doing their first-choice subjects, Prof Smithers suggested, which may "lower some of their performance".
But Prof Smithers did say that pressure on schools to ensure pupils meet a set standard could counteract changes within exams.
This year schools must ensure that at least 40% of pupils get five or more Cs at GCSE including English and maths, as well as meet national progress measures. Those that do not meet the threshold, which is reported in performance tables, are considered to be failing.
"It's a fact that schools are very concerned about meeting the floor target. And they have found new ways of gaming the system, like the multiple entries," he said.
"All the effort going on from schools to push up the marks will come up against the determination of the Government and Ofqual to secure standards.
"Most of the indications are that it will go down."
Last month Ofqual said it was expecting to see a " small drop in achievement" in science following a move to toughen up the qualifications after a 2009 report by the regulator found that the courses were too easy. T his is the first summer that results will be given for the revamped GCSEs.
"The exams have been toughened up, so we almost certainly will see some lowering in science of A* and A grades," Prof Smithers predicted.
Ahead of tomorrow's results concerns have also been raised about multiple and early entry, with figures showing that tens of thousands of pupils took papers for more than one maths GCSE last summer, while the numbers sitting key exams before age 16 has soared. These strategies could also have an impact on the overall pass rate.
Prof Smithers suggested that many of the pupils being entered for maths exams multiple times were not those " being sped on to continue to AS or A-level".
"They are going to be pupils for whom the school wants to secure as many goes as possible so they can get the crucial C grade. GCSEs are almost more important to schools than they are to pupils because if more than 40% do not get five A*-C grades including English and maths the school will be under scrutiny.
"So schools are very keen, possibly more for their sake than their pupils, that pupils get at least a C, and if you take exams lots of times you might fluke a C."
Brian Lightman, general secretary of the Association of School and College Leaders, said: "In the context of enormous uncertainty and piecemeal changes to exams, schools are doing everything they can to give students the best possible chances.
"This is not about gaming, it's about ensuring that we do the best for students in our care."
He added: "The accountability system drives practice but there is no doubt that GCSEs, particularly in core subjects, are key to many careers and of vast importance to students."