79Thus, although Article 2(b) and (d) of the CEOS expressly provides that temporary staff may be engaged to fill a permanent post, it also stipulates that that may occur only temporarily. Moreover, the second paragraph of Article 8 of the CEOS provides that temporary staff are not to be engaged for more than four years, but their contracts may be renewed not more than once for a maximum period of two years. At the end of that time, they must cease to be employed as temporary staff, either by termination of their employment or by their appointment as officials in accordance with the Staff Regulations. That exception to the principle that permanent posts are to be filled by appointing officials may apply only in order to meet the needs of the service in a given case (see, to that effect, Joined Cases 341/85, 251/86, 258/86, 259/86, 262/86, 266/86, 222/87 and 232/87 van der Stijl and Cullington v Commission [1989] ECR 511, paragraphs 28 and 33). Furthermore, it may apply only if the institution has a vacant permanent post previously prescribed by the budget (Martínez Páramo and Others v Commission, paragraph 77 above, paragraph 97).