Britain's Trident nuclear missile system misfired during a test last month when a missile crashed into the ocean near the submarine that launched it, The Sun newspaper reported on Wednesday.
The Sun said the first-stage boosters on the missile, which was equipped with dummy warheads, did not ignite during the Jan. 30 test, which took place off the coast of Florida.
The Ministry of Defence (MoD) said an anomaly had occurred during the test.
"As a matter of national security, we cannot provide further information on this. However, we are confident that the anomaly was event specific, and therefore, there are no implications for the reliability of the wider Trident missile systems and stockpile," the MoD statement said. "The UK's nuclear deterrent remains safe, secure and effective," it added.
A 2016 test of the system - reported by The Sun to be the previous most recent such test - also ended in failure when the missile was reported to have veered off course.