Martens, 77, became the leader of the Christian Democrat party in 1972, the Flanders News reported, and became prime minister in 1979.
Martens was forced to focus on the economy in his early years as prime minister, the Flanders News said, and he played an important role in shaping Belgium into a federal state.
He handed over the role to Jean-Luc Dehaene in 1992.
In 1994, Martens entered the European Parliament and led the European People's Party until this year.
Martens was married to Miet Smet, who served as a junior minister in several of his administrations, the Flanders News reported.
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