Premier League champions City and Girona are both part of the City Group, while Jim Ratcliffe who owns 25 percent of FA Cup winners United also controls Nice through his Ineos company.
UEFA said it had accepted the admission of the four clubs into next season's European competitions, after previously opening proceedings into them for potential conflict with its ownership rules, "following the implementation of significant changes by the concerned investors in Girona FC and OGC Nice".
"The clubs were able to demonstrate that such changes brought them into compliance with the multi-club ownership rule," UEFA's Club Financial Control Body (CFCB) said in a statement.
"No one is simultaneously involved, directly or indirectly, in any capacity whatsoever in the management, administration and/or sporting performance of more than one club participating in a UEFA club competition.
"No one has control or decisive influence over more than one club participating in a UEFA club competition.
"The significant changes made to the ownership, governance, and financial support of the concerned clubs, substantially restrict the investors' influence and decision-making power over more than one club."
It said City Group, which owned a 47 percent stake in Girona, and Ineos had transferred their shares to "independent trustees through a blind trust structure established under the supervision of the CFCB First Chamber" until July 2025.
UEFA said that the clubs had also promised not to make any new player transfers to each other, either permanently or on loan, until September 2025 and would not cooperate commercially or on coaching or scouting.
When the stakes are returned next July, the clubs will again need to take steps to satisfy UEFA rules.
Last year, UEFA investigated three cases involving Toulouse and AC Milan, English club Aston Villa and Portuguese club Vitoria Guimaraes, and Premier League Brighton and Union Royale Saint-Gilloise of Belgium.
