Charles is expected to undergo a medical within the next 24 hours after Fulham struck an agreement worth £30m, understood to be made up of £27m guaranteed plus a further £3m in add-ons, with reports elsewhere breaking the figure down slightly differently as £26m upfront and £4m in performance-related bonuses.
The final go-ahead had been held up while Fulham waited for clearance from Manchester City, who retain matching rights and a sell-on percentage from Charles's original €12.3m move to Southampton in 2023, having come through City's own academy before that transfer.
The 22-year-old Northern Ireland international had been keen to leave St Mary's for a chance at Premier League football after the club's Spygate saga cost them and their teammates a place in the Championship play-off final last season.
It was a scandal that saw Southampton docked points and Charles's own promotion hopes dashed despite a strong individual campaign that included six goals and two assists in 38 appearances, among them a decisive strike in the play-off semi-final against Middlesbrough.
Charles, who has previously captained his country and made more than 30 senior appearances for Northern Ireland, has grown into one of the more coveted young midfielders outside the Premier League as a result.
Leeds United had three separate offers rejected for Charles over the course of the summer, reportedly worth £20m and £23m before a third approach also fell short of Southampton's valuation, but it is Fulham who have ultimately won the race for his signature after Crystal Palace also entered the picture briefly earlier in the window, drawn in part by the prospect of Europa League football under Pierre Sage.
Charles will link up with new head coach Álvaro Arbeloa at Craven Cottage, joining a rebuilding project following Marco Silva's departure for Benfica after five steady seasons in charge, during which Fulham never finished below 13th in the Premier League.
The midfielder is understood to be poised to sign a five-year contract, giving Fulham a long-term option in central midfield as they continue reshaping their squad this summer, which has also seen Gonzalo García, César Palacios and Jonah Kusi-Asare arrive at Craven Cottage while Raúl Jiménez and Harry Wilson departed.