More secondary school pupils are being taught in classes of more than 30 than they were two decades ago, teachers’ leaders’ claim.
But County Hall bosses insist they are on top of a predicted rise in demand and may increase admission numbers in some areas.
According to latest figures, Lancashire has 127 schools which are either full or exceeding their capacity.
Data from the Department for Education revealed that of the county’s 640plus schools, 117 primaries and 10 secondaries were either at capacity or overcrowded last year, and the same number of schools were full or overcapacity in 2016-17.
The National Education Union has accused the Government of “inadequate planning” for the increase in pupil numbers and is calling for a change in rules to allows councils to open new schools.
Andrew Morris, assistant general secretary of the NEU said: “This is an unacceptable state of affairs. Our children and young people only get one chance for an education.”
More than 400 new secondary school places are in the pipeline and Edwina Grant, Lancashire’s executive director for education and children’s services, said school places planning was complex.
She added: “The county council has a statutory duty to ensure that we have the right number of school places and our officers have a very good track record of doing that.
"We have a strong track record in forecasting the future need for school places, and are recognised as being one of the best authorities in the country. Our figures for pupils attending one of their three preferred schools are higher than the national average.
"We are working with the Regional School Commissioner inviting Trusts and providers to tender for new schools throughout Lancashire.
"Where there is a need for additional places, we have built extra classrooms.
"We have provided over 3,500 additional places in the last 10 years and we will continue to monitor this situation on an ongoing basis.
"We are currently looking at increasing admission numbers at schools in some parts of Lancashire, recognising that the need for secondary school places is now growing.
"After a period of lower demand, some schools have spare capacity and we are looking at ways of utilising this efficiently."