He hopes Wednesday's redundancies will be the last at the club.
"It was a purely economic decision that we had to make," Birch said.
"We have a very small amount of money that we have to stretch out for the rest of the season and David and his fellow directors have joined with other redundancies that we have had to make.
"I feel sorry for them as they have all worked so hard over the past few months in difficult circumstances, but we have to try and get to the end of the season.
"That's it now in terms of the redundancies. We don't expect any more; we do it in one fell swoop and hopefully that will take us through to the end of the season."
Pompey's squad is thinning by the week with injuries also limiting manager Michael Appleton's options, and Birch revealed that the Football League will not allow them to bring new players in.
"The cash available is worse than we first feared," said Birch.
"Some of the bits of assumptions that were made in terms of income to take us forward we've now taken a more conservative view on that. We've got to conserve the cash to make sure it takes us through to the end of the season.
"The Football League will not let us bring any players in and they don't want us to have a competitive advantage when we still owe clubs money.
"If we have less than 14 fit players then they will allow us to bring someone in."
Birch is open to the idea of a fans consortium buying the stricken club, adding: "We won't rule out at all a supporters trust buying the club if that is the best game in town and certainly we will look at it. They can bid on their own or along with another interested party."
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