The Sachsgate affair crossed the boundaries of comedy and taste, the director-general of the BBC has told the Leveson Inquiry.

Mark Thompson said the incident, when comedians Russell Brand and Jonathan Ross left a series of lewd messages on the voicemail of actor Andrew Sachs in 2008, had been a “very serious lapse of editorial judgement”.

He said it highlighted a lack of compliance in “that part of the BBC” and caused the corporation to add new guidelines on “intimidation and humiliation”. Brand and the controller of Radio 2, Lesley Douglas, resigned following negative press reaction at the time.

He added: “Certainly that was a programme that went far, far, far beyond the line”.

Thompson told the inquiry that the core of the BBC’s editorial mission was to deliver “the most trustworthy and accurate journalism that we can”. He said secret filming was a last resort for BBC programmes and was only justified when “clear prime facia evidence [showed] that there was some criminality or wrongdoing at work”.

He added: “We don’t do any investigations into people’s private lives for their own sake”.

He said the BBC only received two to three privacy complaints a year, and were the subject of genuine mistakes rather than “wilful intrusion of any kind”.

The director-general said the BBC had conducted an internal investigation into phone-hacking and this was “necessary and appropriate” in light of allegations made against the News of the World. He told the inquiry: “I have a very high level of confidence in saying that these things did not happen at the BBC”.

He added: I believe that the guidelines and values of the BBC are clearly against it”.

He said private investigators were sometimes used by the corporation but usually for “security and surveillance”, including protecting journalists at work in dangerous situations. He also said investigators helped to track down individuals to deliver “right of reply” letters, and that Steve Whittamore had been used by the BBC to locate a suspected paedophile on an aeroplane.

Thompson told Lord Justice Leveson that the “core of culture” at the BBC was “incredibly strong” and did not need to be changed. He said BBC journalists could teach him “one or two things about journalistic standards” and their “probity and conviction was not to be questioned”.

He said the BBC receives over a million “contacts” from the public every year, and 240,000 of these are complaints.

He added: “I don’t believe we’ve lost a defamation action in court for a decade”.