Historians say Hitler Youth members who visited the city in the 1930s were "unlikely" to be Nazi spies.

In the years leading up to the Second World War, the security services grew suspicious of holidays taken by the Hitler Youth.

MI5 believed the German teenagers were using their holidays as a cover for espionage.

Dossiers released this week show MI5 was so concerned about the danger posed by "spyclists'' – groups of touring German cyclists – it ordered police to report whenever a group arrived in the country.

In October 1937, 24 German teenagers from Hamburg, marched from the Clock Tower to lay wreaths at Leicester war memorial, in Victoria Park.

The event, which included German songs and Nazi salutes, was featured on the front page of the Leicester Mercury.

Leicester historian Austin J Ruddy, who is writing a book about German bombing raids in Leicestershire, said: "It is unlikely that the Abwehr – German military intelligence – would rely on Hitler Youth schoolchildren for intelligence gathering of potential military targets, especially as they were so conspicuous.

"We do know, however, that information on Leicestershire's industries, detailing their location and output, was freely available via trade journals and catalogues. A more likely explanation for the bombing of industrial targets, including in Leicester, was the series of pre-war spy flights by German civil aircraft equipped with ground-mapping cameras."

MI5 concerns that German cyclists were carrying out covert spying operations in Britain appear to originate from a May 1937 article in the now-defunct Daily Herald newspaper.

Under the headline "Nazis must be spyclists", it warned that the Nazi Cyclists Association had issued orders to its members who were spending holidays abroad, in which it encouraged them to memorise the locations of landmarks, the construction of bridges and the width of streams.

The visit to Leicester by 24 members of the Hitler Youth, was led by Hermann Fahle. They stayed for two weeks.

A picture of Herr Fahle giving a Nazi salute was also featured in the Leicester Mercury article about the visit, published on Thursday, October 21, 1937.

Leicester was hit eight times by German bombing raids, which included the destruction of the pavilion in Victoria Park, on November 20, 1940.

In the same year, a gasworks in Aylestone Road, was targeted by the Luftwaffe.

Professor of European history at De Montfort University Panikos Panayi said: "The Hitler Youth were seen as much more than just a scout movement, its aim was to sculpt the next generation of German soldiers.

"But I think it's unlikely they were here for any sort of reconnaissance. It would be difficult to draw any conclusion without knowing why they were here and who invited them. Most of the Hitler Youth training would have taken place in their own backyard, not in other countries."

Read more on the curious episode of the Hitler Youth in Leicester in the March edition of the Leicestershire Chronicle. It also features the story of Oswald Mosley's Blackshirts in the city.