SpaceX will reportedly no longer be sending a pair of space tourists to circle the moon this year.

The flight -- set up by the rocket company founded by billionaire Elon Musk -- was scheduled for late 2018, but this has been delayed, according to The Wall Street Journal.

The reason for the delay is unclear.

The flight was announced in February 2017, with SpaceX saying that two unidentified private citizens had put down a "significant deposit" for the trip and that other flight teams had expressed interest in taking a similar journey.

The plan was for the tourists to fly on a Dragon Crew spacecraft launched from Earth by a Falcon Heavy rocket.

"SpaceX is still planning to fly private individuals on a trip around the moon and there is growing interest from many customers," company spokesman James Gleeson wrote in a statement. "Private spaceflight missions, including a trip around the moon, present an opportunity for humans to return to deep space and to travel faster and farther into the solar system than any before them, which is of course an important milestone as we work toward our ultimate goal to help make humanity multi-planetary."

On Friday, Musk challenged Boeing's assertion that it would put the first human on Mars before SpaceX does.

First published on June 4, at 3:24 a.m. PT.
Update, 8:11 a.m. PT: Adds SpaceX statement.