Under the proposals, new entrants to teaching after 2006 would be made to work an extra five years to get a full pension.
Susan Johnson, head of pensions for the 160,000-strong Association of Teachers and Lecturers, said the teaching profession could now look less attractive, particularly to mature career changers.
She said: "It's already proving difficult to recruit enough people into teaching and having to wait another five years to retire may discourage more people from entering the profession.
"No one chooses teaching to get rich, but the guarantee of a good pension at the age of 60 has always been an incentive to take up a teaching career."
One-third of students on postgraduate teaching courses are more than 30 years of age. She said: "These proposals may particularly affect the number of mature entrants to the profession."
Eamonn O'Kane, general secretary of the 200,000-member National Association of Schoolmasters Union of Women Teachers, shared her concerns.
He said: "The current problems will be compounded if the Government pushes ahead.
"Already far too many teachers leave to take up less pressured jobs. A higher retirement age will force many more to reconsider their career choices.
"With levels of ill-health and retirement high in the profession, as well as some teachers losing the energy to cope with the demands of the classroom as they get older, raising the retirement age to 65 could result in more teachers' lives being blighted by stress.
"There may be a minority of teachers who wish to stay in their jobs beyond 60, which could be encouraged on a strictly voluntary basis."
Barry Fawcett, head of pensions at the National Union of Teachers, said:
"Clearly raising the retirement age would be a retrograde step. There are issues about how realistic it is to ask teachers to carry on after they are 60."
The Teacher Training Agency, however, expects any pension changes to have a minimal effect on recruitment.
Mary Doherty, director of teacher supply and recruitment for the agency, said: "Teachers do enjoy a good pension package at the moment and I am sure that will continue to be the case."
Avoid pensions panic, page 48