In England, education secretary Michael Gove has signalled the end of modular GCSEs in an effort to end what he calls the "culture of resits".
But in Wales, where a review of 14-19 qualifications is underway, schools will be free to offer modular or linear GCSE courses until at least 2014, the Welsh Government announced this week, prompting fears of a two-tier exam system.
Robin Hughes, national manager for awarding body OCR Cymru, warned that the move risked undermining public confidence if the Welsh version is seen as a soft option.
"This poses a huge number of questions because it gives you the potential for a two-tier parallel system with linear and modular GCSEs here in Wales and only linear GCSEs in England," Mr Hughes said. "Will the modular GCSE be classed as the 'Welsh GCSE'? Between now and 2014 it could be a pretty confused picture to say the least. We should do everything we can to mitigate the very real risk that a 'Welsh GCSE' might be characterised as less rigorous."
From September 2012, students in England will sit all their exams at the end of their two-year GCSE courses, instead of sitting some along the way in "bite-sized chunks" and re-sitting any they fail. The Westminster Department for Education said the move would "restore rigour" to the exam system and reduce pressure on the timetable.
The Welsh Government also has concerns over modular GCSEs, and their future is being considered as part of a wide-ranging review of qualifications for 14 to 19-year-olds. But it says any changes it imposes will be based on evidence and will not be implemented until at least September 2014.
"This means that, should awarding organisations wish to continue to offer the current GCSE specifications to centres to learners in Wales, they are free to do so during this interim period," a spokesman for the Welsh Government said.
"Alternatively, if centres wish to transfer to the revised specification they, likewise, are free to do so. Either way, the existing 40 per cent terminal assessment requirement will remain for all GCSEs, as will the overall quality assurance systems to standardise outcomes."
Mr Hughes said the Welsh Government's attitude to consultation stood in stark contrast to Mr Gove's "hasty, unilateral decision" in England. But the resulting two-tier system will need to be carefully explained. "We will need to consider how to communicate the changes to schools, parents, learners and important stakeholders like HE (higher education) and employers," he said.
The Cardiff-based WJEC exam board said it was looking at the regulatory implications of the announcements from both Westminster and Cardiff Bay, but that it fully supported the Welsh Government.
Gareth Pierce, WJEC chief executive, said: "We have no difficulties with the principle of allowing a modular approach to continue in Wales, even if it is discontinued in England, and we look forward to working closely with regulators in both countries to ensure comparability of standards.
"For some subjects, we believe that the approach has been very successful, - for example, in science where this structure is well-established and has contributed to the increased take-up of the three separate sciences in both Wales and England."
Gareth Jones, secretary of teaching union ASCL Cymru, said heads in Wales would welcome the flexibility. But, he added, it was crucial that action was taken to ensure standards remained consistent. "(Michael) Gove's proposals in England may suit some students but it may disadvantage average and lower ability students," he said.
The gap between the top-performing GCSE pupils in Wales and those in the rest of the UK has widened for the fifth year in a row.
Entrants achieving A*-C grades this year
Pupils gaining five GCSEs (grades A*-C) this year