Macedonian President Gjorge Ivanov refused on Tuesday to sign off on a deal with Greece over changing his countrys name to “North Macedonia.” Greece and Macedonia last week signed a historic preliminary agreement to rename the small Balkan nation, ending a row that has poisoned relations between the two neighbors since 1991. The move, however, did not sit well with Ivanov, who vowed not to approve the deal. In a statement Tuesday, his office argued that the agreement is unconstitutional. His strong words could signal difficulty ahead for the government, which must still amend the constitution and has pledged to put the name change to voters in a referendum. Prime Minister Zoran Zaev said he will resign if the agreement fails to get support in the promised referendum. "Macedonia has no plan B," Zaev told 1TV. Ivanovs decision will delay but not derail the deal that Macedonias parliament has already ratified. Under the countrys constitution, if lawmakers meeting for a second time once again approve legislation that the president has refused to sign off on then the president can no longer block it. Parliament speaker Talat Xhafer told The Associated Press that lawmakers will probably repeat the vote next week. The agreed deal with Greece will require several more steps, including a referendum this autumn in Macedonia, before it can be fully implemented. The deal was also met with opposition in Greece. On Sunday, some 4,000 people protested in Thessaloniki against the name deal, police said. The protesters, mainly from the far right, brandished banners reading "politicians traitors." They marched towards the offices of the governing parties of SYRIZA and ANEL,with some turning violent and hurling objects. Police fired tear gas to disperse the protesters. The name accord aims to start unraveling one of the worlds longest diplomatic disputes, which began 27 years ago with Macedonias declaration of independence, but dates back centuries. Since 1991, Athens has objected to its neighbor being called Macedonia because it has its own northern province of the same name, which in ancient times was the cradle of Alexander the Greats empire -- a source of intense pride for modern-day Greeks. Greek Prime Minister Alexi Tsipras has been accused of treachery by Greek hardliners, and last week defeated a vote of censure against his government amid protests and clashes with police outside parliament.