UPDATE 1-Greece condemns Turkey's decision to convert Hagia Sophia into mosque

(Adds background, former Greek minister's quote)

ATHENS, July 10 (Reuters) - Greece condemned a decision by Turkey on Friday to convert Istanbul's Hagia Sophia museum into a mosque, saying it would have repercussions not only on relations between the two countries, but on Turkey's ties with the European Union.

Turkish President Tayyip Erdogan said on Friday that the first prayers would be held in Hagia Sophia on July 24, after declaring the ancient monument was once again a mosque following a court ruling revoking its status as a museum.

Erdogan said the nearly 1,500-year-old Hagia Sophia would remain open to Muslims, Christians and foreigners, but added that Turkey had exercised its sovereign right in converting it to a mosque and would interpret criticism of the move as an attack on its independence.

"Greece condemns in the most intense manner the decision of Turkey to convert Hagia Sophia into a mosque. This is a choice which offends all those who also recognise the monument as a World Heritage Site. And of course it does not only affect relations between Turkey and Greece, but its relations with the European Union," Mitsotakis's office said in a written statement.

Former Greek foreign minister Dora Bakoyannis and Mitsotakis's sister tweeted that Erdogan had "crossed the Rubicon" with its decision, effectively moving itself way from the Western world.

UNESCO said on Friday its World Heritage Committee would review Hagia Sophia's status and that Turkey's decision raised questions about the impact on its universal value as a site of importance transcending borders and generations. (Reporting by Angeliki Koutantou and Renee Maltezou; Writing by Michele Kambas; Editing by Nick Macfie)