Former Greek Prime Minister Costas Simitis, who played a pivotal role in guiding Greece into the eurozone in 2001, passed away on Sunday at the age of 88 at his summer residence in the Peloponnese.

“With sorrow and respect, I bid farewell to Costas Simitis, a distinguished and honorable political opponent, as well as the Prime Minister who led Greece through significant national milestones,” said Greek Prime Minister Kyriakos Mitsotakis in a statement.

The Greek government announced four days of national mourning, with the state covering the costs of his funeral, according to the Greek state news agency.

Simitis, a law professor and advocate of reform, led the PASOK socialist party from 1996 to 2004, serving as prime minister throughout his tenure. Recently vacationing near Athens at his Peloponnese home, Simitis was rushed to the hospital unconscious early Sunday morning, where his death was confirmed, the director of Corinth Hospital reported.

During his time in office, Simitis implemented fiscal reforms that reduced Greece's budget deficit and public debt, enabling the country to join the eurozone.

In 2012, three years into Greece's debt crisis, Simitis authored Derailment, a book critiquing how Greek politicians and the EU managed the crisis. In it, he also accused the European Commission of overlooking overspending under his conservative successor.