The plant is one option in an internal competition between Ford's European plants to win production of the next Transit.
An internal memo, leaked to the Southern Daily Echo, said Ford had found economic and operating factors which made it difficult to achieve the investment necessary at Southampton to continue current production levels.
In the memo, plant manager Martin Chapman urged everyone to do what they could to secure the plant's future "otherwise the Transit will be built elsewhere".
Ford told AM: "As is usual at this stage in a product cycle, Ford is discussing with employee representatives and union officials plans for future model investment at Southampton plant.
The spokesman said investment discussions are ongoing, and will next formally reconvene in September.