Porto could be relegated to the Portuguese second division after being formally notified yesterday that they and their club president, Pinto da Costa, are under investigation for instances of match-fixing in 2003-04. That season Jose Mourinho, who went on to coach Chelsea, led the team to victory in the Champions League.

A brief statement on the club website read: "FC Porto and their president have been notified by the Portuguese Professional Football League (LPFP) of the accusations and investigations concerning matches between FC Porto and Estrela da Amadora, and Beira-Mar and FC Porto."

Porto won the first match 2-0 and drew 0-0 at Beira-Mar. Both results are under investigation over allegations of corrupting the referees. The case, which is referred to in Portugal as "the golden whistle", has been monitored for nearly four years, with close collaboration between the LPFP and the Portuguese legal system.

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Porto have sent the accusations to their lawyers and have five days to submit their defence.

The club could be docked three points from this season's total – they currently lead Benfica by 16 points – or they could be relegated. Da Costa could be banned for two to 10 years if found guilty of corruption; attempted corruption would carry six months to two years.

The president was charged in 2005 for three offences of active corruption and corrupt practices but was released because of a lack of evidence.

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