"For decades the BBC denied that job applicants were subject to political vetting by MI5"
But in fact vetting began in the early days of the BBC and continued until the 1990s
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".
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.
The Christmas tree was eventually dropped in 1984 because it was said to attract too much attention.