"Such information hands the advantage to the terrorists. It is the gift they need to evade us and strike at will. Unfashionable as it might seem, that is why we must keep secrets secret, and why not doing so causes such harm."
Parker did not name The Guardian or Snowden in his comments but said that the information gleaned about terrorists and "the detail of the capabilities we use against them" was what gave intelligence services their "margin of advantage" -- a margin, he said, that is now "under attack."
A former head of GCHQ and adviser to 10 Downing Street, Sir David Omand, also last week said in an interview with The Times of London that leaks by Snowden of intelligence documents represent "the most catastrophic loss to British intelligence ever."
However, in an editorial for The Guardian published Monday, a former director of UK public prosecutions, Ken Macdonald, argued that Parker had used "foolish self-serving rhetoric" to argue against greater transparency by security agencies.
Reporting based on Snowden's leaks has also caused diplomatic tensions for the United States with both Brazil and Mexico.