Pompey administrator Andrew Andronikou fears HMRC will lodge a protest against the agreement between the club's creditors before the deadline of midnight on Thursday.
The move would mean the club staying in administration until the appeal is heard, and Pompey would therefore remain under the transfer embargo due to Football League rules.
Andronikou told The News: "We believe the HMRC are going to appeal, that is the indication we are getting. It's going to be an irritant and, to put it simply, the embargo will stay for a longer period.
"As long as there is an appeal process, the registration embargo will continue to be in force for the foreseeable future.
"However, we expect any appeal hearing to be held in October or November. Before then, if the squad is below 20, we can still bring in players, regardless of the embargo. That is allowed and is something we are looking at."