The number of teachers attending science and maths training courses has fallen dramatically following the introduction of workforce reforms, a key government adviser has warned.
Enquiries about specialist courses promoted by the National Centre for Excellence in the Teaching of Mathematics have dropped by half since September, when teachers were guaranteed they would only have to "rarely cover" lessons.
Attendance at training run by the National Science Learning Centre is down by 25 per cent during the same period. The rarely cover rules are designed to stop the practice of teachers being asked to step in regularly for absent colleagues. It means that schools must pay for extra staff to cover lessons instead.
An unintended consequence is that schools are now less willing to release staff for training courses, according to Professor John Holman, the Government's national director for STEM (science, technology, engineering and maths).
Professor Holman, who is also director of the National Science Learning Centre, said: "The situation is really serious. It is a good regulation and the effect on training is unintended, but it has reduced opportunities for teachers to get out of school.
"This also includes field trips and attendance at events run by industry. It could be that headteachers are being overzealous, and everything might improve. We are very much hoping this will be the case."
Mark McCourt, director for delivery at the National Centre for Excellence in the Teaching of Mathematics, said: "There are less and less people looking for training opportunities. I think a lot of people have got the idea rarely cover means they can't leave school."
An investigation into the issue will be held next month by the Commons' education select committee.
Barry Sheerman, chair of the committee, said people in the profession were "very worried" about rarely cover. "There are amazing continuing professional development (CPD) resources, but schools are reluctant to use them because they don't want to organise cover and pay for supply teachers at great expense," he said.
John Bangs, head of education at teaching union the NUT, said: "We are now witnessing the unforeseen consequences of rarely cover, and we have plenty of evidence it is leading to teachers being prevented from going on CPD courses."
Martin Ward, deputy general secretary of heads' union the Association of School and College Leaders, said financial pressures might be exacerbating the problem.
"It's not that school leaders are reluctant to have their staff trained, but the impact of rarely cover has had a difference and made it that little bit more difficult," he said. "It's brought out the cost of training, which has remained hidden. We all know it's not a good thing to cut, but it's also optional and this is a choice heads are making in these current times."
A DCSF spokesman said: "There is absolutely no reason why, with careful planning, the introduction of rarely cover should interfere with teachers being released to attend CPD activities and school trips. Where a teacher has been timetabled to take a lesson for an absent colleague attending an external training event, this is not classed as cover."