Ministers said most schools were ready to ensure teachers had 10 per cent of their timetable free for planning, preparation and assessment (PPA).

The Welsh Assembly government was not aware of any schools unable to deliver on PPA. But a spokesman said any schools without a plan in place “should contact their LEA as quickly as possible to discuss extra help”.

Only 14 schools in England have officially asked the Government for help with the final phase of the workforce deal. But the National Association of Head Teachers said the real number of schools that would struggle to provide PPA time was likely to be higher.

The workload deal, agreed two years ago, has already stopped teachers from carrying out some administrative tasks and covering for colleagues for more than 38 hours a year.