With Middlesbrough slumping to a 4-3 defeat to Fulham and Norwich failing to see off Rotherham, the league-leading Hornets can only be caught by Bournemouth, who play on Monday.
Watford, who now boast five successive wins, ground out a workmanlike success before a crowd of 28,841 at the Amex Stadium to return to the top flight, eight years after their relegation from the Premier League.
Slavisa Jokanovic's side took the lead in the 29th minute when Troy Deeney rifled in his 21st goal of the season from close range.
The Seagulls should have taken something from the game and defender Joe Bennett wasted an opportunity to grab them a point late on, before Czech substitute Matej Vydra sealed the points with a cool finish in the last minute of stoppage-time
A hat-trick from Lee Gregory in Millwall's 3-3 draw with Derby was not enough to lift the Lions out of the relegation zone
Millwall looked on course to leapfrog Rotherham, who were deducted three points for fielding an ineligible player on Friday, when they were 3-1 up after 50 minutes, but a 70th-minute penalty from Chris Martin and Jeff Hendrick's strike five minutes from time handed play-off hopefuls Derby a valuable point to keep Millwall in 22nd, one point behind the Millers
Derby's late equaliser handed a lifeline to Wigan, who are three points adrift of safety after their 1-0 defeat to Wolves, which leaves Kenny Jackett's side eighth in the table, two points away from the play-off places.
Brentford are also two points behind sixth-placed Derby after seeing off Reading 2-0
Middlesbrough's hopes of automatic promotion were dealt a blow by Ross McCormack, who scored three goals including a stoppage-time winner as Fulham won 4-3 at Craven Cottage
Norwich's promotion hopes also took a hit as Rotherham scored a late equaliser in a rousing 1-1 draw
Elsewhere Freddie Sears scored a priceless 83rd-minute winner as fifth-placed Ipswich remained in the play-off zone with an edgy 2-1 victory over Nottingham Forest
Huddersfield and Blackburn played out a 2-2 draw, while Birmingham recorded a 1-0 victory over Charlton
Leeds beat Sheffield Wednesday 2-1 in a hotly-contested Yorkshire derby at Hillsborough and Cardiff beat basement club Blackpool 3-2
Source : PA
Source: PA