The growing trend for private takeovers of education has led to fears that teachers' pension entitlements may be left to the whim of employers.
In a letter to Education Secretary David Blunkett, Peter Smith, general secretary of the Association of Teachers and Lecturers, said teachers may not be able to continue contributing to the Teachers' Pensions Scheme if they work in an area run by an external company, appointed by the local education authority.
In the case of education action zones and in areas where the Government has forced the authority to contract out, ministers have insisted that contractors keep up contributions for thei staff to the Teachers' Pensions Scheme.
But where outsourcing is undertaken voluntarily, it is open to the authority to set its own terms. Now the union wants the Government to make it obligatory for all contractors to pay into the scheme.
From April teachers' employers' contribution to the scheme will be 7.4 per cent of salary.
Sue Johnson, head of the ATL Pensions Department, said there was a fear that private firms may choose not to contribute in order to maximise their profits.
But a Department for Education and Employment spokesman said: "Where outsourcing is voluntary by the local education authority, the department considers it would be right for the contract to cover membership of the pensions scheme, but we cannot insist on it."