The government has no plans to carry out its own investigation into unidentified flying objects (UFOs) and whether they pose a threat to the UK, a defence minister has told parliament.
Baroness Goldie fielded questions in the House of Lords on the UK's response to a Pentagon report which was released last week.
The long-awaited document said of the 144 sightings reported by military pilots since 2004, all but one remain unexplained and they "probably lack a single explanation".
It said that while there were "no clear indications that there is any non-terrestrial explanation" for the objects, this prospect was not ruled out.
"UAP (unidentified aerial phenomena) clearly pose a safety of flight issue and may pose a challenge to US national security," the report said.
Former Conservative Party treasurer Lord Sarfraz raised the issue in the Lords on Wednesday.
"For decades people who have been concerned with UFOs have been dismissed as fantasists," he said.
"But now the US director of national intelligence, who oversees 17 intelligence agencies, has published a report saying that data on UFOs is inconclusive.
"The report offers several possible explanations.
"It does not rule out that these could be military aircraft with very fast capabilities or even extraterrestrial phenomena.
"Either way can the minister reassure members of the public that the Ministry of Defence (MoD) takes reports of unidentified objects in our airspace very seriously?"
Baroness Goldie said in response that the MoD "holds no reports on unidentified aerial phenomena" but "constantly monitors UK airspace to identify and respond to any credible threat to its integrity, and is confident in the existing measures in place to protect it".
She added: "The MoD deals with actual threats substantiated by evidence. The government continues to take any potential threat to the UK seriously."
Lady Goldie told peers later in the debate: "The MoD has no plans to conduct its own report into UAP because in over 50 years no such reporting had indicated the existence of any military threat to the United Kingdom."
Tory peer Viscount Ridley said: "Unidentified does not mean suspicious. Does the minister recognise the US report referred to says there is no clear indication that there is any non-terrestrial explanation for the 144 sightings?
"The idea that in an era of mobile phone cameras, drones and frequent travel there could possibly be alien spaceships whizzing about undetected in our atmosphere on a regular basis is not I think very plausible.
"It's much more likely that these blurred images have boring explanations, alas."
Lady Goldie said in response: "The UK air defence community detect and monitor all flying air systems 24 hours a day to provide an identified air picture as part of the UK's national security posture and our commitment to the integrity of Nato airspace."