Italy international Diamanti, who scored the winning penalty in Sunday night's shoot-out victory over England in Kiev, signed for the Hammers in August 2009 from Livorno for ?6million, but he was allowed to move back to Italy after just one season in the English Premier League.
It is, though, the player's subsequent transfer to Bologna in a co-ownership deal following Brescia's relegation to Serie B that has left the Hammers incensed.
In July 2011, West Ham asked for Diamanti's registration by the Italian Football Federation to be suspended as they had not yet been paid the latest installment of the ?2million transfer fee.
West Ham co-owner David Gold said on Twitter: "We sold Diamanti to Brescia because he was desperate to return to Italy.
"He was then sold to Bologna. We are suing Brescia for the money."
Source: PA
Source: PA