asked the Secretary of State for the Home Department the number of new cases in the last three years in which he has given authority for the letters of individuals to be opened during their passage through the post?

It would not be in the public interest to give this information.

Can the right hon. Gentleman give any indication as to whether the number has been large or not in order that the public may know whether this practice has been carried out upon an extensive scale?

The word "large" is a comparative term. All I can say is that when the public interest demands this practice, I carry it out, and not unless.