Hundreds of UFO reports are believed to be made by pilots each year, but reports over the last six years going back to 2011 are not being released.
UFO investigators tried to obtain the files under the Freedom of Information Act, but the CAA is using EU legislation from 2014 to block the release of the “occurrence”, The Sun reports.
The CAA said: “Occurrence information can only be used for the purpose of maintaining or improving aviation safety, and the release of occurrence information to the general public or the media, including in response to Freedom of Information Act (FOIA) requests, is not permitted.
“However, if you require occurrence information for the purpose of maintaining or improving aviation safety you are able to make an application to the CAA.”
The CAA has said that it is to protect the identity of pilots reporting the sightings.
However, investigators have rubbished the explanation, saying the FOIA allows for such personal details to be redacted from what is released.
Dr David Clarke of Sheffield Hallam University’s department of journalism exposed the lack of transparency.
He has before been allowed access to earlier versions of the files, but his latest FOI for the current information was blocked.
He told The Sun Online: “These have been collected and logged by the CAA since at least 1976.
“Indeed in 2012 the chief executive of Britain’s National Air Traffic Control Services, Richard Deakin, admitted in a BBC Radio 4 interview that his agency received reports of UFOs from civil aircrew somewhere in the world every month.
“But then they seem surprised that curious individuals might want to see details of these incidents using Open Government legislation.
“Now they are using a piece of European Regulation to block public access to these records.
“The only conceivable reason for this change of policy is embarrassment on the part of the aviation industry.
“It does not want to admit that its pilots do occasionally report things in the sky that are difficult to explain.
“To improve public confidence in air safety, the authorities should be proactively promoting open access to records of this type.”
The CAA claimed that the files are available, but only to those who would use the information to improve safety in the sky.
They are, therefore not available to the general public or media.
A spokesman said: “The Mandatory Occurrence Reporting (MOR) scheme requires individuals and organisations within the aviation industry to report safety occurrences to the CAA, with the intention that these reports are used to constantly improve safety levels.
“Information held by the CAA under the MOR system may be made available, for the purpose of improving aviation safety, subject to completing this application form.”
The Ministry of Defence (MoD) previously investigated the UFO phenomenon until 2009.
Most of its records have now been released via The National Archives, but three files have been held back.