The Blues have decided against keeping the 19-year-old forward beyond his current 12-month scholarship deal.
It is not yet known if Bognor will seek to offer Lethbridge a contract for next season, or if the player will look for a chance higher up the leagues.
Lethbridge has made three appearances for the Pompey first-team in the EFL Trophy – scoring once in a 2-2 draw with Oxford.
Lethbridge has spent a fair chunk of the past two seasons on loan at Nyewood Lane, scoring 20 goals in 71 games for the Rocks but also suffering a number of interruptions to his progress through injuries.
The last time Pompey released a striker who was on loan at Bognor, it was Dan Smith - and the Rocks then took him on full-time.