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Folkestone will be playing Ryman League Premier Division football next season.
Invicta were 2-1 winners over Godalming in the Division 1 South play-off final at the Buzzlines Stadium on Saturday.
Josh Vincent put the home side in front with a fine diving header after 18 minutes.
But their joy was short-lived when Bill Williams headed home the equaliser soon after.
Invicta hit back to ensure they went into the break ahead when Simon Austin a scored following a Jimmy Jackson free-kick.
Godalming had a couple of penalty appeals early in the second half, before Chris Wells sent a good chance over the bar.
The visitors appeared to have been successful with their third penalty shout, but the referee changed his mind about awarding a spot-kick after consulting his assistant.
The victory capped a remarkable season for Neil Cugley’s side who recovered from having a 10-point penalty deduction.