Academies are free to expand without the permission of local education authorities and are unfettered by the local priorities of the Learning and Skills Council.
St Mary's sixth-form college, in Middlesbrough, faces competition from both King's academy and Unity academy, each of which caters for 11 to 18-year-olds. They replaced schools which only took pupils up to 16.
Don Lillistone, principal of St Mary's, said: "One of the key points in opposing the establishment of the King's academy was that, as an 11-18 school, it broke the stated pattern of provision.
"It is risible to establish a post-16 planning body, the LSC, and at the same time allow ad hoc 11-18 academies that are independent of the planning body to be established in areas with an 11-16 pattern of provision."
A third academy is to be created from the existing Macmillan college and it has been given extra capital funding of pound;10 million to ease the transition.
Miriam Stanton, principal of Bede sixth-form college, said: "By 2008, it is expected that there will be no sixth-form college in Middlesbrough."
Julian Gravatt, director of funding and development at the Association of Colleges, said the problem is not restricted to the north of England. "No college is going to invest in the future without looking at the numbers," he said.
The Office for Standards in Education, in its report on Middlesbrough, said: "There is a lack of coherent planning for post-16 provision in Middlesbrough.
Mr Gravatt added: "London colleges are facing the challenge of academies that offer specialist 16-18 courses, with a ton of government money and some private-sector support."
A recently-retired principal said colleges need a level playing field to co-operate with other training organisations.