Lord Baden-Powell founded the world-famous Scout movement
MI5 suspected that members of the Hitler Youth were using cycling tours of Britain as a cover for spying, declassified files from 1937 show.
Those concerns appear to have stemmed from newspaper coverage.
Records also show that the founder of the Scout Movement, Lord Baden-Powell, was invited to meet Adolf Hitler.
Despite the invitation there is no evidence Baden-Powell met Hitler, although he did meet with some members of the Nazi hierarchy.
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.
MI5 was sceptical about the authenticity of these instructions because it could not trace their source in German publications.
However, the agency still collected dozens of reports of groups of uniformed Hitler Youth who toured Britain before the outbreak of World War II.
Lord Baden-Powell met Hartmann Lauterbacher, chief of staff of the Hitler Youth and Joachim von Ribbentrop, to London 19 November 1937.
Von Ribbentrop, who arranged the meeting for Baden-Powell, was Germany's ambassador to Britain at the time and became Hitler's foreign minister the following year.
Baden-Powell, who attended with a Girl Guides leader, seems to have been impressed by the Nazi officials.
The next day he wrote to von Ribbentrop, saying: "I am grateful for the kind conversation you accorded me which opened my eyes to the feeling of your country towards Britain, which I may say reciprocates exactly the feeling which I have for Germany.
"I sincerely hope that we shall be able, in the near future, to give expression to it through the youth on both sides, and I will at once consult my headquarters officers and see what suggestions they can put forward."
In a report on his meeting, Baden-Powell described von Ribbentrop as an "earnest" and "charming" man.
He wrote: "I had a long talk with the ambassador, who was very insistent that the true peace between the two nations will depend on the youth being brought up on friendly terms together in forgetfulness of past differences.
"He sees in the Scout movement a very powerful agency for helping to bring this about if we can get into closer touch with the Jugend (Youth) movement in Germany.
"To help this he suggested that if possible we should send one or two men to meet their leaders in Germany and talk matters over and, especially, he would like me to go and see Hitler after I am back from Africa.
"I told him that I was fully in favour of anything that would bring about a better understanding between our nations, and hoped to have further talks with him when I return from Africa."