By Agence France-Presse
Directors Phil Lord and Christopher Miller left the project due to creative differences, Lucasfilm president Kathleen Kennedy said.
Lord and Miller "are talented filmmakers who have assembled an incredible cast and crew, but it's become clear that we had different creative visions on this film, and we've decided to part ways," Kennedy said.
"A new director will be announced soon," said Kennedy, who is also the film's producer.
In a statement, Lord and Miller also cited "creative differences".
"Unfortunately, our vision and process weren't aligned with our partners on this project. We normally aren't fans of the phrase 'creative differences' but for once this cliche is true," they said.
"We are really proud of the amazing and world-class work of our cast and crew," they added.
The surprise announcement came as filming is underway at London's Pinewood Studios for the untitled movie, which is scheduled to come out several months ahead of "Star Wars: Episode VIII" in December 2018.
Variety magazine -- which said the directors had been fired -- cited an anonymous source saying that the directors chafed at Kennedy's tight control on the set.
"It was a culture clash from day one," the source told Variety. "She didn't even like the way they folded their socks."
"Kathy, her team and [writer/producer] Larry Kasdan have been doing it their way for a very long time. They know how the cheese is made and that's how they want it made," the source said. "It became a very polarizing set.
In the movie, Alden Ehrenreich stars as Solo, a role that made Harrison Ford a Hollywood superstar in the original 1977 movie and its sequels.
The movie also stars Woody Harrelson, Thandie Newton, Emilia Clarke and Donald Glover as Solo's pal Lando Calrissian.
The film is set in the period before the original 1977 movie, which was released as "Star Wars" but was later retitled "Episode IV: A New Hope."
This is the second of a series of standalone films that began with "Rogue One," released in December, which earned $1 billion worldwide.
Lord and Miller took over the Han Solo directing job after co-directing the box-office hits "The Lego Movie" and both "21 Jump Street" and "22 Jump Street."
Lucasfilm, founded by filmmaker George Lucas, is the production company behind the "Star Wars" and "Indiana Jones" franchises. It was bought by Disney in 2012.