The SNP pledged in its manifesto ahead of the 2021 election to reduce contact time to 21 hours per week, but EIS general secretary Andrea Bradley said the Government has “delivered absolutely nothing”.

Members will be able to vote from Wednesday until January 14 on potential action as a result, with the union’s leadership pushing for teachers to back both strike action and action short of a strike.

The Scottish Government said it is “disappointed” the ballot has been opened.

Any action would be likely to be taken ahead of next year’s Holyrood election and could damage the prospects of the SNP if a resolution is not reached.

“In opening this ballot, we are reminding the Scottish Government and Cosla that it is essential that they keep their promises to Scottish education, Scotland’s teachers and Scotland’s learners,” Ms Bradley said.

“This ballot is a chance for Scotland’s teachers to show both their employers and the Scottish Government their strength of feeling, and to compel them to work constructively together and act on teacher workload.

“I would urge all eligible EIS members to look out for their ballot paper arriving in the post, and to cast their vote in this hugely important ballot.

“The EIS executive strongly recommends to members that they should vote yes to strike action and yes to action short of strike.

“We must send the type of strong and united message that is impossible for the politicians to ignore.”

The patience of both teachers and the union, Ms Bradley added, was “at an end on this issue”.

“We are committed to working with teaching unions and Cosla to agree our approach to delivering a reduction in class contact time at pace.

“Ministers respect union members’ right to withdraw their labour, but are disappointed that the EIS has taken this action while these constructive discussions are ongoing.”