"For decades the BBC denied that job applicants were subject to political vetting by MI5"
BBC:
But in fact vetting began in the early days of the BBC and continued until the 1990s
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The fear was that "evilly disposed" engineers might sabotage the network at a critical time, or that conspirators might discredit the BBC so that "the way could be made clear for a left-wing government".
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If staff came under suspicion only after they had been employed by the BBC or applied for transfer to a job that needed vetting, an image resembling a Christmas tree was drawn on their personal file.
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The Christmas tree was eventually dropped in 1984 because it was said to attract too much attention.