Recruitment adverts for GCSE and A-level markers placed on buses and even in the pages of cruise holiday magazines may not be enough to help solve a staffing shortage, the head of a major exam board has warned, amid calls for reform of the system.

OCR has said it will need to hire an extra 5,000 staff to mark exams, to cope with changes instigated by Michael Gove during his time as Education Secretary, which have been dismissed by teachers’ unions as “poorly thought-through” .

The reforms, which switch the emphasis from coursework marked by teachers to end-of-course exams, will start to put pressure on exam boards to recruit extra teachers from next year – with the full impact being felt by 2019.

Mark Dawe, the chief executive of OCR, told The Independent “we struggle every year” to recruit examiners but next year may see a need for an extra 5,000 staff.

“Given we have about 15,000 markers at the moment, it wouldn’t surprise me if that’s going to push on to over 20,0000,” he said. “It may be more.”

The Government has responded robustly, saying if it is not confident of getting enough markers the board should pull out of the contract and let someone else offer the qualifications.

Subjects which create the most difficulties include religious studies – where the number of entries has dramatically increased in recent years but which has fewer qualified teachers – as well as history, English and geography.

Mr Dawe also pointed out that the board had lost more than half the staff who had volunteered for marking the classic civilisation paper when it switched to online marking – possibly because it was too modern a concept for them.

In order to combat shortages, the board has advertising in a P&O cruise magazine for the first time this year, after one of its examiners went on a cruise holiday and noticed how many teachers – both serving and retired – choose to holiday at sea. It has also placed adverts on Birmingham buses on routes going past schools.

But Chris Keates, the general secretary of the National Association of Schoolmasters Union of Women Teachers, also warned that the Gove reforms had created “an enormous challenge” to recruit more markers.

She added that the qualifications reform had been “poorly thought-through” and “driven by ideology not evidence”, with too little thought for the resources needed to implement the reforms.

A spokeswoman for the Department for Education said in response: “Our new exams have been developed with subject experts, industry and universities and are being phased in to give schools time to prepare.

“If an awarding agency like OCR chooses to offer a particular qualification, it is incumbent on them to make sure they have sufficient high-quality markers.”