In what the newspaper describes as the only test of a British missile in four years, the unarmed missile veered off in the direction of the U.S. mainland, the exact opposite of the Royal Navy's intent.
In a BBC television interview Sunday, May declined to answer four times on whether she knew about the test firing.
Instead, she said: "I have absolute faith in our Trident missiles. When I made that speech in the House of Commons, what we were talking about was whether or not we should renew our Trident."
As media speculation intensified, a spokeswoman told reporters Monday that the prime minister did in fact know of the failure.
"On taking office, the current prime minister was briefed on a range of nuclear issues, including this,"she said.
The Ministry of Defense has not denied reports of a bungled test but issued a statement claiming that Trident's "capability and effectiveness . . . is unquestionable".
Expanding, the ministry said: "In June the Royal Navy conducted a routine unarmed Trident missile test launch from HMS Vengeance as part of an operation which is designed to certify the submarine and its crew. Vengeance and her crew were successfully tested and certified, allowing Vengeance to return into service. We have absolute confidence in our independent nuclear deterrent. We do not provide further details on submarine operations for obvious national security reasons."
The U.K has 4 Trident submarines, with one permanently armed and at sea.
Correction: The headlines on this story were revised to clarify that the missile was not armed.