Watford has won promotion to the English Premier League after beating Leeds United 3-0 in the playoff final at Cardiff's Millennium Stadium.
It joins champions Reading and Sheffield United as the three promoted teams. Watford had finished the season in third place in the Championship, with Leeds in fifth.
American defender Jay DeMerit put Watford in front midway through the first half when he met an inswinging corner at the far post with a firm header.
Watford doubled its advantage 12 minutes into the second half when a James Chambers shot was deflected off Leeds's World Cup-bound US midfielder Eddie Lewis, hit the base of the near post and went in off diving goalkeeper Neil Sullivan.
Darius Henderson put the finishing touch to an emphatic victory with a penalty seven minutes from time after Shaun Derry brought down striker Marlon King.
The victory to Watford, who had one season in the Premier League in 1999/2000, is worth close to STG40 million ($A99.38 million) made up largely of expected television revenue in the top flight, according to British media reports.