Broadwater's title celebrations were put on hold after suffering their first defeat in 29 games spanning the best part of two seasons.
East Preston held their nerve when chasing a large total to claim a two-wicket victory.
The leaders went into the game needing 30 points to be declared champions. They now have to try again at home to third-placed Crawley Eagles this weekend.
Another defeat could leave the door open for Southwater, who won by seven wickets at Clymping.
East Preston skipper Andy Reeves was delighted with the victory against Broadwater.
He said: “We took an early wicket but it started to fall apart for us as they piled on the perssure with their batting.
“They declared with a view to getting a result, which is the right way to play cricket.
“What I wasn’t aware was quite how weak their bowling attack was this week. Joel Kingsbury, who normally bats at eight, opened and batted well as we chased it down, getting there in the gloom with 2.4 overs to spare.”
Broadwater set 254-6 with wicketkeeper/batsman Richard Luker scoring 85. In reply, Kingsbury hit 51, Jack Sunderland 76 and Chris Williams 59 not out.
Reeves is now hoping for a good finish to the season for his fourth-placed side. He said: “It started off with two good wins but then we had a lean patch. I gave the team a rollicking after we were bowled out for 80 against Horsham Trinity and we won the next three games.
“We have got to look to see if we can finish third. We are certainly looking up rather than down.”
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