The Ionian sea in Greece has been approved by the nation's supreme court from six to 12 nautical miles.
The decree covers the maritime zone from the Ionian Sea islands to Cape Tenaro at the southern tip of the Peloponnese.
The decree says the Greek government reserves the right to exercise similar rights in other regions notably the Aegean.
In August, Prime Minister Kyriakos Mitsotakis said the government was planning to submit a bill to double Greece’s territorial waters in the Ionian Sea.
In future Greece could also extend its territorial waters in other maritime areas, he added.
In the mid-1990s, Greece similarly attempted to extend its territorial waters in the Aegean Sea to 12 miles but scuttled the plan after Turkey declared such a move would be a casus belli or cause for war.
This declaration on the part of Turkey is a blatant violation of the fundamental principles of the UN Charter on refraining from the threat or use of force (article 2, paragraph 4), on the peaceful resolution of disputes (article 2 paragraph 3) and on good neighbourly relations and peaceful coexistence (Preamble).
The withdrawal of the casus belli against Greece has been included in the basic criteria set for Turkey’s accession to the European Union as part of its obligation to fully respect international law and good neighbourly relations, one of the fundamental principles on which the European Union is founded. It is obvious that a candidate member state cannot threaten war against another state, not least an EU member state and future partner.