The number of secondary students in Lancashire will rise by 14 per cent over the next five years.
The latest forecast from the Department for Education shows Lancashire County Council will have 75,480 students between 11 and 16 years old in 2023-24, 9,512 more than this year.
The 79,142 secondary places available in 2016-17 in Lancashire would be enough to meet demand in five years, unlike in 71 English councils where students demand is growing at a higher pace than schools capacity.
Besides Lancashire, there are more places than students in 65 other local authorities. The department's data measures total school capacity but is not broken into age ranges so, according to teachers unions, there may be under-capacity in specific student years groups which is not revealed by the total number of places.
Local government chiefs said that many councils will face a secondary school "emergency" unless the Government allows local authorities to open new schools or expand academies.
England's birth rate went up steeply in the early 2000s, leading councils to add about 600,000 extra places in primary schools since 2010. That birth bulge is now feeding through secondary school.
Coun Anntoinette Bramble, chair of its Children and Young People Board, said: “Councils need to be given the powers to help solve this crisis. As a starting point, they should be allowed to open new maintained schools and direct academies to expand.
“It makes no sense for councils to be given the responsibility to plan for school places but then not allowed to open schools themselves. It is only by working with councils, rather than shutting them out, that we can meet the challenges.”
Nansi Ellis, assistant general secretary of the National Education Union, said: “The Government’s determination to create a market for education through the introduction of free schools and the expansion of the academy programme and grammar schools has undermined the ability of local authorities to plan and provision of school places.
"We face a secondary school places emergency unless the Government sees sense and gives councils the powers to open schools or direct academies and free schools to expand."
A DfE spokesperson said: “This Government has driven the largest creation in school places in two generations and by 2020, there will be one million more new places across the school system than there were in 2010.
“We are spending £23 billion by 2021 to ensure every child has access to a good school place and since 2010, 43,000 fewer pupils are being taught in overcrowded schools. Our latest admissions data shows that 93.8 per cent of children received offers from one of their top three choice of secondary school last year.”
Manchester will need to expand its secondary schools the most in England. According to DfE data, it will need 12,797 new places in five years - about 11 new schools.