Teachers will next month debate whether to take industrial action to force Scottish ministers to reduce class sizes in key subjects in secondary school.

The call from members of the Educational Institute of Scotland (EIS) follows a decision by the last Scottish Executive to drop a commitment to reduce secondary class sizes to a maximum of 20 for S1 and S2 maths and English.

In a letter to councils in December, the executive said its commitment would be met if average class sizes were 20, arguing that it had always been its intention to allow schools flexibility, rather than holding them to an absolute maximum.

However, the decision provoked anger with Ronnie Smith, general secretary of the EIS, accusing ministers of reneging on their election promises of four years ago.

In June, following a motion by the Edinburgh branch of the union, delegates at the EIS annual conference in Perth will debate whether to take strike action in August.

However, it is unclear whether the motion will receive wider support given the political map of Scotland has been radically redrawn following the Holyrood elections last week.

The national executive of the EIS has not yet decided whether to support or oppose the motion and the SNP, which won the election by the narrowest of margins, believes it is premature to start threatening strike action over a policy which was not theirs.

However, Colin Mackay, Edinburgh branch secretary, said his members felt very strongly that they had been "welshed on" and felt industrial action was appropriate.

He suggested one option was for a work to rule which would result in teachers accepting the first 20 pupils into their classes and sending the rest to the headteacher.

"If we object to teaching more than 20 pupils, the rest will have to be taught elsewhere," he said.

An SNP source accused the EIS branch of "putting a gun" to the head of the new administration.

"There's an issue as to what extent a new government should have to take the consequences for the failures of the previous one," said the source.

"This party has pledged to reduce class sizes, but what action can be taken in the short space of time before teacher contracts are signed? They have to be reasonable about what they can expect."

An executive spokeswoman said: "We are unable to comment as we do not yet have a new administration in place."

Meanwhile, another teaching union - the National Association of Schoolmasters Union of Women Teachers (NAS/UWT) - is to debate a call to ban mobile phones from the classroom at its conference in Dundee next week.

The move is being considered against the backdrop of an increasing number of bullying cases of both pupils and teachers involving mobile phone cameras where footage is posted on the internet.