Albion are deep in relegation trouble after Leicester powered to the win they needed to clinch promotion and return to the top of the table.
The Seagulls are four points adrift of safety with three matches left after fellow strugglers Stoke beat Wimbledon 2-1 at the Britannia Stadium.
Former Albion and Leicester forward Ade Akinbiyi was one of the scorers for the Potteries club.
Sheffield Wednesday drew 0-0 with Grimsby in their relegation battle at Hillsborough.
Albion could be relegated at Withdean on Monday, but only if they lose to Wednesday and Stoke win at Coventry.
That gloomy scenario for Steve Coppell's battlers is in stark contrast to the joyous scenes which greeted the final whistle for Leicester and their manager Micky Adams.
He guided Albion to the Third Division title two years ago and went over to applaud the 2,000 travelling fans from Sussex.
A full house of nearly 32,000 at the Walkers Stadium saw the Seagulls start brightly before falling behind in the tenth minute.
Former Albion loan forward Paul Dickov crossed to the near post for the diving Muzzy Izzet to head home his fifth of the season.
The match was effectively put beyond Albion's reach on 44 minutes when the unmarked Jordan Stewart thundered a header into the roof of the net from Izzet's corner, his fourth of the campaign.
It must have been enfuriating for Coppell to see his side concede a header from a corner again, just as they had against Preston at Withdean last Saturday.
Albion could count themselves unfortunate to be two behind at half time.
Ian Walker was the busier of the keepers in the opening 45 minutes, making smart saves from shots by Paul Brooker early on and then Paul Watson.
Coppell brought on Nathan Jones for Dean Blackwell, Charlie Oatway for Brooker and Graham Barrett in place of Bobby Zamora in the space of eight minutes deep into the second half, but Albion never really looked like getting back into the game.
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