Campbell left the band he helped create in 2008 and he was followed out the door by keyboardist Mickey Virtue, while a third founding member, Astro, quit last year.
The trio have now announced that they will be regrouping to tour and record a new album together.
They have already lined up a slot at the Bearded Theory music festival in England in May, and are working on new music in the studio.
A statement from the musicians reads: "The three founding members have reformed and reunited to bring you the true sound of UB40. Ali, Astro and Mickey are currently in the studio working on a new album."
The reggae group has faced numerous problems over the last few years, including rumours of a rift over management decisions and money troubles following the failure of the band's now defunct record label DEP International.
Four members were declared bankrupt in October 2011, including Astro, real name Terence Oswald.
It is not yet known if news of the original trio's reunion will affect the current line-up of UB40.