Parthenon marbles, why does the British Museum not have a valid title

On March 12, 2021, British Prime Minister Boris Johnson declared that the 2,500-year-old sculptures of the Acropolis in Athens are «lawfully owned by the Trustees of the British Museum» and «they will never be returned to Greece». The Hellenic Minister of Culture, Lina Mendoni, Ph.D. in Archeology and exponent of the New Democracy, the center-right party that has ruled the country since July 2019, replied saying that Greece is now in possession of additional evidence to support their argument.
For Dr. Elena Korka, archaeologist, Honorary Director-General of the Antiquities and Cultural Heritage Directorate at the Greek Ministry of Culture and Sports, and President of the Greek chapter of the Blue Shield, the best proof in Greece's possession is a letter from Lord Elgin, dated July 28, 1811, in which he asks his successor, Ambassador Adair, to change the meaning of a sentence in a letter from the officers of the Ottoman Empire, translated by the British Embassy chief translator Bartolomeo Pisani. Lord Elgin suggests that Sir Adair may propose the change of the sentence «the Porte absolutely denied you (Lord Elgin) having any property rights on these marbles» to «the Ottomans in Athens did not have the authority to let the marbles be removed». After some back and forth with Ottoman officers, offering money and gifts, Sir Adair managed to have the documents changed and the rest is history.

The most recent evidence was found about two years ago in the British Library by Dr. Elena Korka and, today, Greece is ready to use it. According to Dr. Korka «the position of the United Kingdom cannot be maintained forever». 50% of the architectural elements and marbles of the temple erected to Athena Parthènos, surviving to the present day are in the British Museum in London, while 45 % are in Athens. The remaining 5% is dispersed between public European collections, such as the museums of Copenhagen, Wurzburg, Munich, Vatican City, Palermo, Vienna, and Cambridge. To date, only the University of Heidelberg, Germany, has returned one frieze.