Montenegro has declared Serbia's ambassador persona non grata for interfering with the country's internal affairs.
Following the announcement on Saturday, Serbia declared persona non grata the Montenegrin ambassador to Serbia and gave him 72 hours to leave the country.
Montenegro's interior ministry said that despite warnings not to interfere into internal affairs, the Serbian ambassador said the 1918 session of the parliament in which Montenegro's sovereignty was abolished was "liberating".
The tiny Adriatic state of 600,000 was a kingdom until 1918 when it abolished its sovereignty to join Yugoslavia.
It stayed in a union with Serbia after Yugoslavia fell apart in the early 1990s and declared independence in 2006.
The outgoing government that expelled Vladimir Bozovic will be succeeded by a new pro-Serb cabinet of the parties that won elections in August.
The pro-Western Democratic Party of Socialists led by President Milo Djukanovic, which steered Montenegro through the violent collapse of the former Yugoslavia in the 1990s, lost the parliamentary election last week.
It will not be able to form a government for the first time since 1990.
Australian Associated Press