The ex-Pompey academy striker has turned down Bognor’s offer of a new deal to stay at Nyewood Lane.
He is thought to be looking for a new club – although Rocks manager Jack Pearce has said the door is still open for him to come and talk further to the club.
Lethbridge has been a hit at Bognor since first joining them on loan from Pompey in 2018.
He has made 72 starts for the club and eight appearances off the bench, scoring 25 goals – and he set up Doug Tuck’s winning goal in the 2019 Sussex Senior Cup final.
The Rocks had hoped the Hampshire-based striker would rejoin for 2021-22 but that now looks unlikely.
Pearce said: “We wish him well if he finds a new club.
“As he is under 24 a compensation fee will have to be agreed with us.”
Another striker, Jordy Mongoy, has committed to another Rocks campaign in the Isthmian premier division, joining the likes of Amadou Tangara, Harvey Whyte, Joe Cook and James Crane in agreeing to stay at the Nye Camp.
Meanwhile there have been one or two minor changes to the Rocks’ programme of pre-season friendlies.
The visit to Littlehampton on Wednesday, July 7, will kick off at 7.30pm while the match at Horndean on Saturday, July 24, will have a 2pm start.
And the visit to Havant & Waterlooville will be on Saturday, July 17, with a 3pm start.