West Ham's blistering first-half performance ensured a significant victory which virtually secures their Premier League status but leaves Watford in major trouble.
David Moyes' side picked up a crucial win to move six points clear of the relegation zone with two games remaining and, with a goal difference that is also significantly better than their rivals', the Hammers' place in the top flight seems assured.
But the picture looks much bleaker for Nigel Pearson's men, who lie just three points outside the drop zone and face Manchester City and Arsenal in their remaining games.
The hosts scored twice in the opening 10 minutes through the in-form Michail Antonio, who swept home and Tomas Soucek's bullet from Jarrod Bowen's cross.
Declan Rice curled in a sublime third before half-time and although Troy Deeney pulled a goal back at the start of the second half, Watford could not conjure a miraculous turnaround.
Relive West Ham's win over Watford'Promise less and deliver more' - West Ham do just that
Former Manchester United and Everton boss Moyes was appointed for a second spell as West Ham boss in December, with the side just one point above the relegation zone.
Preserving the club's top-flight status will have been the aim and Moyes will achieve that objective barring a freak set of results over the coming days.
West Ham's goal difference currently stands at -13, considerably better than Bournemouth's -25 and Aston Villa's -27 so even if they are caught on points there will need to be a colossal goal swing for them to be dragged down into the Championship.
Prior to the match, Moyes had asked his players to "promise less and deliver more" and they did just that in a terrific first-half showing that eased any nerves.
Forward Antonio, fresh from scoring four against Norwich in the previous game, took his tally to nine for the season with a calm, left-footed finish to set them on their way.
Soucek's flying header and Rice's delightful goal ended the game as a contest, despite there being nearly an hour still to play.
West Ham stalwart and captain Mark Noble made his 500th appearance for the club and will be particularly overjoyed with the result having previously been part of the side that was relegated to the Championship at the end of the 2010-11 campaign.
Hammer blow for Hornets
The sides occupied the two places immediately outside the relegation zone and the result was a damaging blow for Watford and their ambitions of staying up.
Heading into the game, the Hornets had shown decent form by picking up back-to-back victories but a desperately poor first half could prove fatal.
Pearson's men remain perilously placed - they need four points to be certain of staying up but finish the season with a daunting home fixture against Pep Guardiola's City and a trip to Arsenal.
Ismaila Sarr saw a low drive well saved by Lukasz Fabianski in the opening period and though they improved in the second half with Christian Kabasele heading wide and Deeney pulling a goal back, the damage had been done.
More to follow.