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Dartford face the National League South play-offs again after losing the title race on the final day of the season.
Havant & Waterlooville were promoted as champions thanks to a 3-2 home win against Concord Rangers.
Dartford won their game at Bognor Regis but were denied automatic promotion by goal-difference in the most dramatic climax imaginable.
There were recalls for Danny Harris and Ryan Hayes on the south coast as Dartford, who arrived on the back of eight straight wins, made two changes to their line-up.
Keaton Wood, who left the Darts earlier this season, started for the relegated home side.
But although Bognor are going down, they appeared determined to go out with a fight and should have taken an early lead.
Harris slipped and Justin Amaluzor was played clean through only to roll his shot narrowly wide of the far post.
Dartford were understandably playing with urgency but in their haste began to misplace passes just as moves were coming together.
Calvin Davies turned and fired a shot wide for the Rocks while at the other end, Hayes and Lee Noble both failed to hit the target.
Amaluzor had Ronnie Vint worried and when the centre-half was caught in possession, he sent another effort past the post.
Hayes then came up with a peach of a cross which was headed straight at goalkeeper Dan Lincoln by Darts skipper Elliot Bradbrook.
The ground fell quiet as news came through that Havant were ahead and things almost got worse when the visitors had to scramble a Harvey Whyte header off the line.
Danny Mills forced a save from Lincoln after Alfie Pavey nodded a deep cross into his path but Darts continued to look shaky at the back.
Ollie Pearce dragged a shot wide as the home side found gaps at will, although Dartford came very close to breaking the deadlock just before half-time.
Bradbrook's header was kept out by Lincoln and the follow-up effort from Vint was blocked on the goal line.
But Dartford's defensive frailties were still evident after the break. Thomas Scutt played Jimmy Muitt into space in the penalty area and only a good block from Vint kept the ball out of the net.
Mark Onyemah fired over but the home side were able to get in behind the Darts defence far too easily.
They had claims for a penalty waved away and should have taken the lead moments later but picked the wrong pass.
Mills thought he'd scored when he headed Hayes' right-wing cross towards the far corner but Lincoln pulled off a brilliant save to keep the ball out.
But the keeper was beaten just before the hour when Hayes stood up another terrific delivery and Mills scored with a downward header.
The atmosphere transformed in an instant, especially when news of a Concord goal came through, and the visitors piled forward.
Substitute Tom Murphy had a shot tipped over by a flying Lincoln and forced another good block with his next shot.
But still Bognor posed a threat going forward and with so many Dartford players committed upfield, you couldn't take your eyes off the contest - or the news from along the coast.
Onyemah pulled off a vital block before Bradbrook skewed a shot wide as play continued to ebb and flow.
The incredible seemed possible when Dartford made it 2-0, Vint rifling a great shot into the bottom corner from distance.
And it should have been three straight from kick-off but Pavey blazed over the bar.
For a few minutes Dartford were going up as champions, with Concord having fought back to 2-2 at Havant. You could now hear Darts fans all around the ground with more than 600 having made the trip from Kent.
But after Bognor pulled a goal back through Pearce's penalty, a last-gasp goal from ex-Dartford striker Jason Prior won the title for Havant to leave Tony Burman's players devastated.
Bognor: Lincoln, Whyte, Davies, Tuck, Block, Wood, Gilot (Swallow 79mins), Scutt (Charman 70mins), Amaluzor, Pearce, Muitt. Subs not used: Sekajja, Nelson, Boughton.
Dartford: Schotterl, Harris, Vint, Bonner, Onyemah, Hayes (Collier 85mins), Noble (Brown 82mins), Bradbrook, Pugh (Murphy 57mins), Mills, Pavey. Subs not used: Ofori-Acheampong, Ibrahim.
Attendance: 1,017.