Kanu has departed Fratton Park, leaving Dave Kitson, Tel Ben Haim and Liam Lawrence still on the books, but his representatives still want a reported £3million from the club in lost wages and Birch has asked for a tribunal to intervene.
Birch has warned unless the club's remaining senior players either reached agreement over unpaid wages or left the club, Pompey will go out of business.
"[Luke] Varney, [David] Norris, [Greg] Halford and [Erik] Huseklepp have all effectively walked away from what they are owed," Birch told the Portsmouth News.
"There are substantial sums in there but they have made compromises to leave the club with years left on their deals. They have also agreed for their deferrals to be paid over a certain period of time. It's a terrible time for everybody but they have helped the club and I'm sure fans will thank them.
"That means there are now three players left because Kanu effectively walked away at the end of last week after submitting his 14-day letter. He is still claiming, however, so we will file a submission to the Football League, with it going to a tribunal hearing to decide on the issue. Hopefully they can hear it as quickly as possible."