The body of George Baldock, a British-born defender for Panathinaikos and the Greek national team, was discovered Wednesday in the swimming pool at his home in Glyfada, a southern suburb of Athens, according to police sources.

The cause of death is currently unknown.

Authorities attempted cardiopulmonary resuscitation on the 31-year-old former Premier League player but were unable to revive him, a police official reported. His time of death was estimated to be around 10 p.m. (7 p.m. GMT).

Baldock, who had Greek ancestry and earned 12 caps for the national team, joined Panathinaikos in May after Sheffield United's relegation from the Premier League. He also played for lower-league sides MK Dons and Oxford United.

Sheffield United said they were shocked and extremely saddened by the death of a popular player who spent seven years at Bramall Lane, with other English clubs sending their condolences.

Baldock made four appearances for Panathinaikos, including a 0-0 draw with local rivals Olympiakos over the weekend, but had not been selected for Greece's Nations League squad for a game against England at Wembley on Thursday.

England and Manchester United defender Harry Maguire, a former Sheffield United player, posted a photograph of Baldock on Instagram with a broken heart emoji and "RIP."

"With deep sadness and pain, the national team and the Hellenic Football Federation say goodbye to George Baldock," the Greek team said on Facebook.

"There are no words to describe the human pain caused by the news of the untimely loss of one of our young people. The moment calls for silence. Condolences to his family. Condolences from his second family. Goodbye."