Greek border services have started covering the letters "MK" on Macedonian car number plates with a sticker of their own, marking a new low in Greek-Macedonian relations.
The Greek liaison office in Skopje confirmed the new practice, saying the country had a right to do this under the 1995 UN interim agreement that regulates relations between the two states.
They say this will affect only vehicles with the new plates containing the letters MK.
Macedonia introduced new number plates in February, saying they were needed to meet EU standards. Unlike the old ones, the new plates feature MK in small letters in the corner.
Macedonia's Interior Ministry said it was surprised by the move and was still considering its reaction.
Relations between Macedonia and Greece have been strained for two decades by the row over Macedonia's name. Greece insists that use of the term "Macedonia" implies a territorial claim to its own northern province of the same name.
Citing the unresolved issue, Greece has been blocking Macedonia’s progress towards both EU and NATO membership. UN-brokered talks to overcome the dispute have so far failed to yield a solution.
The Greek move could potentially have a major impact on tourism, as Greece is a top summer destinations for many Macedonians - and many drivers may object to having their cars tampered with in this way.
In the summer of 2011 there were some 400,000 entries of Macedonian citizens to Greece, records show.
The UN recognised Macedonia under the provisonal term of Former Yugoslav Republic of Macedonia, or FYROM, in 1993 - but most Macedonians strongly resent use of the term.