Such action would still be an option after a ballot but would be less successful from a union point of view since it would mean a loss of national control.

Ironically, the result might be that unions are pushed towards a strategy that would have maximum impact, national strikes.

Targeting specific areas, such as constituencies of Labour ministers, may be another tactic that could involve local ballots.

But legal challenges against unions from authorities and parents opposed to action are also much more likely given the raft of employment legislation.

Any old-style, work to contract would be messy. In theory, teachers could withdraw co-operation from initiatives, refuse to cover for absences and return to a ban on extracurricular activities.

In secondary, they could boycott Higher Still, although many would be reluctant to stop the work they have already put in. They may not want to hurt pupils who have started courses.

Whatever form of action emerges, authorities would probably wish to avoid disciplining teachers - other than by docking pay - but would, by law, be forced to ensure schools carried out duties such as national testing and reporting to parents. Young people have to be adequately prepared for examinations through the curriculum.

Barbara Clark, assistant secretary of the Scottish Secondary Teachers’ Association, said: “We will be seriously considering a whole range of options and will take a decision if it’s necessary. We are aware of the strength of feeling among members about the employers’ offer and the response of the minister. They are very angry.”

“The things you can do are not very different from what you could do before,” Ronnie Smith, general secretary of the Educational Institute of Scotland states, “although there is a degree of uncertainty over what is and what is not strike action or action short of a strike. There has been developing case law since the legislation.”

It would take up to six weeks for any action to take place. Employers have to be given seven days’ clear notice of a ballot and a further seven days’ notice of any action following the ballot. Action has to be within 28 days of the ballot.

Mr Smith accepts that most forms of industrial action, including a work to rule, potentially carry a penalty and all would have to be covered by a ballot. Every member has to be able to vote and ballot questions have to be carefully worded.