Greece's defence minister, who is head of the main coalition partner of Prime Minister Alexis Tsipras, announced his resignation on Sunday ahead of an upcoming parliamentary vote to end a 27-year name dispute with Macedonia.
"The Macedonia issue does not allow me not to sacrifice my post," Panos Kammenos said after a meeting with Tsipras.
"I thanked the prime minister for the cooperation and I explained to him that for this national issue we cannot continue," he said, adding that his Independent Greeks party (ANEL) "is pulling out of the government".
The nationalist ANEL supports the Tsipras administration with its seven lawmakers and has six ministers and junior ministers in the government.
Kammenos had threatened to pull out of the government when the name deal comes to a vote in Athens from the moment Tsipras signed the agreement with Macedonian Prime Minister Zoran Zaev in June. However, some of his ANEL party's MPs remain ambivalent.
Macedonian lawmakers voted late on Friday to rename their country the Republic of North Macedonia but the agreement now needs backing from the Greek parliament to come into effect.
For the Greeks, Macedonia is the name of a history-rich northern province that was the cradle of Alexander the Great's ancient empire.
"Within ten days, in any case as soon as the (Macedonia parliamentary vote) result is notified to us and if we see that everything is in order, we will vote (to approve) the Prespes Agreement," Tsipras told Open TV earlier this week.
The Greek prime minister believes the agreement can be ratified with the 145 lawmakers of his leftist Syriza party but also the votes of the small pro-EU To Potami party as well as some ANEL lawmakers that disagree agree with the position of Kammenos.
The main opposition party of New Democracy is vehemently against the agreement.
Revising the constitution to The deal was signed in the border Prespes region, after months of talks with Skopje.
Tsipras added that a separate vote on a protocol to enable Macedonia to join NATO would be held "not long afterwards".
Greece has previously blocked its neighbour's accession to NATO and the European Union ever since Macedonia broke away from the former Yugoslavia in 1991.
'The Macedonia issue does not allow me not to sacrifice my post,' Panos Kammenos said after a meeting with Prime Minister Alexis Tsipras.