It will be rebranded as Believe from this month across bars, multipacks, ice creams and drinks during the tournament.
Masterfoods says the change will link the brand with the feelgood factor around sporting events. It is backing the move with a £3.7m spend, the brand's biggest support since its relaunch four years ago.
Activity is set to include TV, press, radio and poster ads, direct mail, in-store PoS and promotional deals and links with The Sun and Talksport.
The TV ads will feature a football-obsessed character called Bill Lever who will rally supporters to get behind the England team in the World Cup, while a branded 'Believemobile' will be taking sampling and entertainment to nine UK cities next month and then on to Germany. Consumers will be encouraged to interact with the campaign via a web site.
In Scotland, Wales and Northern Ireland, the 'Believe' slogan will be a general philosophical message.
Trade relations manager Andrea Taylor said: "The campaign and the significant media support will give Mars huge visibility and high-scale presence at this feelgood time."
Masterfoods has also kicked off a £10m ad campaign to support Galaxy, which runs until the end of the year. Targeting the key audience of 25 to 44-year-old women, executions focus on the rituals they follow when eating the chocolate.