"Nearly 4,500 teachers in the university departments and colleges are working on ad-hoc basis which is around half of total faculty positions in the university.

"As per UGC and DU's guidelines, ad-hoc appointments should be made for a short period of not more than four months and proportion of these faculties should not be more than 10 per cent of the total number of teachers but the varsity has developed a culture of appointing ad-hoc faculties even on permanent sanctioned posts for years," the letter said.

"The situation is so critical that in some colleges the proportion of ad-hoc faculties has reached to 90 per cent. Majority of these teachers have been working for 5 to 10 years on ad-hoc basis and in few cases it is more than twenty years," it added.

The teachers, have also sent a memorandum in this regard to the Prime Minister's Office, HRD Ministry and University Grants Commission (UGC).

"We demand that the appointments of those ad-hoc teachers who have been serving the university for years be regularised and they be saved from the discriminatory and exploitative service conditions which are affecting the teaching-learning environment of the premier institution," the letter said.