The Irish National Teachers' Organisation is to direct its members not to co-operate with the Government's JobBridge scheme.
The scheme offers internships to unemployed people who get an additional €50 a week for taking part.
The decision was taken by the union's executive at a meeting today after consultation with members.
The JobBridge scheme was extended to schools in September.
The INTO will now issue a directive to its 32,000 primary teacher members telling them not to participate.
The union says take up of the scheme so far in schools had been minimal. A spokesperson said there was widespread opposition to the programme among members.
The Department of Education has described the INTO decision as regrettable.
In a statement, it said the scheme had benefits for both unemployed teachers and schools and should be viewed as a positive measure at a time of high unemployment.