Headteachers at Catholic schools in Glasgow and North Lanarkshire face possible disciplinary action over letters sent out to parents urging them not to vote for the Scottish Green Party.

Both local authorities have apologised and admitted that the action breached the law banning councils from publishing any material designed to affect public support for any political party.

The Herald has seen letters sent out on official Glasgow City Council notepaper from John Reilly, headteacher of St Mungo's Academy, and under the North Lanarkshire Council imprint from Michael Currie, headteacher of Our Lady's High School in Cumbernauld.

Both letters had identical wording and highlighted the Green Party's opposition to Catholic schools, even although the party had made clear that it was not suggesting any immediate action on this, but a debate on the issue.

The letters appended an article by Michael McGrath, of the Scottish Catholic Education Service, and information about the "save our schools" campaign being organised by the Scottish Catholic Observer.

Both letters concluded: "Please give careful consideration to the enclosed information and use your vote wisely on Thursday 3 May."

Glasgow Council responded by writing to all head-teachers of Catholic schools, warning them: "Sending out a letter about the election would have been inappropriate. By law, authorities are prohibited from publishing any material which appears to be designed to affect public support for a political party."

All schools which sent the initial letter were also ordered to send out a fresh letter to parents and carers, stressing: "The headteacher, in issuing these matters, simply complied with a request, but did so unaware that by law, all local authorities are prohibited from publishing any material which appears to be designed to affect public support for a political party.

"In relation to the elections on May 3, this council and its employees acting in their official capacity are strictly neutral and are expected to act impartially at all times. The views expressed in the headteacher's letter should therefore not be taken to reflect the views of Glasgow City Council but as directing you to the views of the Scottish Catholic Education Service.

"Despite this distinction, these materials may be interpreted as aligning a council employee and the wider council against a political party, in this case, the Scottish Green Party. I would wish you to be assured that this is not the case and that the issuing of such materials was inappropriate and should therefore be disregarded."

A Glasgow Council spokeswoman said: "We take this issue very seriously indeed. Glasgow City Council is clear that this was completely inappropriate and as soon as we discovered this had happened, we took steps to rectify the situation and to ensure it remained an isolated incident."

Murdo Maciver, head of educational resources at North Lanarkshire, said a follow-up letter was issued on Tuesday. "In that letter, we clarified that the issue of these materials by the school was inappropriate and stated that the views expressed did not reflect the views of the council. We also advised that we understood that the views expressed contain inaccuracies in relation to the position of the political party referred to.

"By law, local authorities such as North Lanarkshire are prohibited from publishing any material which appears to be designed to affect public support for a political party and the council and its employees acting in their official capacity are strictly neutral and are expected to act impartially at all times.

"We are currently investigating this matter and will take any appropriate action identified following the investigation."

A spokesman for the Greens said last night: "We are disappointed that our position appears to have been misrepresented by many people and, if public resources have been misused, then clearly that will be a matter for the local authorities concerned."