Withdean have clinched the Combined Counties League title with three games to spare.
They won 1-0 at Raynes Park Vale last night to put themselves out of the reach of rivals AFC Wallingford.
Owen Hill lobbed the goalkeeper for the 41st-minute winner after good work by Sam Francis.
Hill and Jay Pickering both had goals disallowed as Withdean dominated.
They have now completed the double following last week's League Cup final win over Ash.
Boss Alan Pook was delighted. He said: "It has been a long season and the lads have deserved all the success they have got. They have been a credit to Sussex football."
Withdean, who have hauled back a huge points deficit in recent weeks, are away from home in their remaining games.
They plan to be tackling the likes of Lewes, Horsham and Worthing in the Ryman League next season.
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