Members of the GMB union, which represents the city’s 368 school janitors, will decide on Sunday whether to ballot for industrial action. Martin Doran, the union’s regional organiser, said there could be “a major industrial dispute”. He did not believe schools could function without the janitors during the day, while heads would be unwilling to undertake call-outs in the evening.

The dispute, which also involves 100 cleaners, is over differing terms and conditions that the council wants to harmonise. Long-serving janitors stand to lose most, according to the union: 50 of them face a drop in wages of pound;4,500 a year. The average loss is estimated to be pound;80-pound;90 a week.

Glasgow City Council confirmed it was attempting to negotiate janitors’ contracts following the setting of its 1998-99 budget. Discussions with the union have been going on for six months and the council hoped agreement could be reached without disruption to schools.

Mr Doran said the union was not opposed to assimilating “emoluments’’ into janitors’ pay and had suggested a solution to the council. This involved buying out workers’ contract privileges which could cost up to pound;400,000. The deal involved janitors undertaking new duties, such as electrical testing which is usually undertaken by outside contractors. This would save a similar sum for the council, said Mr Doran.