Javier Solana, a former NATO secretary-general and European Union official, wrote the message on his Twitter account Sunday, Haaretz said.
Solana said Sharon, who has been in a coma-like state since he suffered a stroke in 2006, had died. When questioned by Twitter users, he said he received the information from friends in Israel.
When proven wrong, Solana eventually apologized for his mistake, the Tel Aviv newspaper said.
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