The exact reason from dropping the batsman was not disclosed by the England and Wales Cricket Board, although Hales was recently suspended for an undisclosed off-the-field incident.
That was the second time he has been sanctioned under the ECB's disciplinary policy, having been suspended and fined for his part in a street brawl also involving England teammate Ben Stokes in September 2017.
"We have worked hard to create the right environment around the England team," said Ashley Giles, the ECB's managing director of men's cricket, "and need to consider what is in the best interests of the team, to ensure they are free from any distractions and able to focus on being successful on the pitch."
Giles said it was "not the end of Alex's career as an England player" and that English cricket authorities "will continue to aid Alex and work alongside his county club Nottinghamshire to give him the support he needs."
As a result of his first suspension, Hales lost his place in England's ODI side, with Jason Roy and Jonny Bairstow the new opening partnership. Hales was set to be a back-up at the World Cup, which is being played in England and Wales starting May 30.
Before then, England plays ODI matches against Ireland and Pakistan.