The Royal Navy may hold records about possible sea monsters but it does not collect them centrally, it has revealed.

Sailors can note unusual sightings on the ocean waves in their ship's logs, the Navy said.

But they are not required to do so and none of the information is assembled in a central archive devoted to sea monsters.

Any sightings of strange marine animals reported to the Navy by the public are passed on to the UK Hydrographic Office, which provides charts and other navigational services for mariners.

Details of the Navy's policy on giant creatures of the deep emerged in response to a Freedom of Information request.

A marine biologist inquired whether the Ministry of Defence held records about "abnormally large or dangerous sea monsters hundreds of metres under the sea" that had not been revealed to the public.

In reply an official wrote: "The RN (Royal Navy), and MoD in general, does not maintain any form of central repository of information purely devoted to sea monsters.

"Personnel might be inclined to record unusual sightings in ship's logs but there is, as far as we know, no actual requirement for them to do so, and it would be beyond the resource constraints of an FoI request to check every line of every RN log book for any such references since 2005.

"However, the RN does invite people to report sightings of marine mammals, and it's possible this could include unusual sightings.

"These are forwarded to the UK Hydrographic Office at Taunton."

The MoD's stance on sea monsters contrasts with the policy on UFOs it maintained for more than 50 years.

Thousands of reports of strange sights in the skies were recorded by the military's UFO investigation unit until it was shut down on December 1 last year.

Old maps often included illustrations of fearsome sea dragons and serpents and there are many tales of mariners' encounters with weird creatures.

Dr David Clarke, an expert in unexplained phenomena, said the Navy was interested in sea monsters in the 19th century.

The National Archives in Kew, west London, contain several historic Royal Navy files about strange sightings on the oceans, he said.

These include an 1830 report sent to the Admiralty in London by the captain of the ship, Rob Roy, about a "great thundering big sea snake" measuring about 129ft long seen by his crew in the waters near the remote island of St Helena in the South Atlantic.

Another file records how Commander George Harrington, captain of the Castilian, saw "a monster of extraordinary length" rear its head out of the sea, again near St Helena, in 1857.

Dr Clarke, a lecturer in journalism at Sheffield Hallam University, said: "At this time they were exploring areas of the world where they thought there may well have been such creatures living.

"I have looked at some of the ship's logs in the National Archives, and there are instructions about what people should record. Any unusual observations of any kind should be recorded in the ship's log."