The .yu domain was established in 1989 for the Socialist Republic
Websites using the .yu domain extension will cease to be available online from 30 September.
The extension - assigned to the former Republic of Yugoslavia - has been replaced by .rs (for Serbia) and .me (for Montenegro).
Icann - which oversees the assigning of top-level domain names - allowed extra time for sites to make the transition before removing the .yu extension.
It is thought up to 4,000 websites have still not migrated to a new domain.
However, the Serbian National Register of Internet Domain Names has requested a postponement of several months; The Internet Corporation for Assigned Names and Numbers (Icann) is meeting to consider the request.
Icann removed the .yu extension from their list of approved country domain names in 2006.
The former Republic of Yugoslavia was renamed Serbia and Montenegro in 2003, although Montenegro subsequently broke from the union in 2006.
Icann says the .rs and .me extensions are now the appropriate domain names as the Republic of Yugoslavia no longer exists.
Established in 1989, the .yu domain was first assigned to the Socialist Federal Republic of Yugoslavia.
With the break-up of the Socialist Republic at the start of the Balkan wars, the .yu domain was held by newly independent Slovenia but was eventually passed on to the Federal Republic of Yugoslavia in 1994.
Since then, it has been managed by the Yugoslav Domain Registry at the University of Belgrade.