Ceri Thomas denied that the briefing of the students had been "shambolic" and rejected claims that students had been forced to run unacceptable risks during undercover filming of the investigation.
He said: "We think the risks as we explained them to the students were justified.
"But I need to be absolutely clear that if we had any suggestion that lives were at risk or anything approaching that - either the BBC team's lives or the lives of the students - then we wouldn't have gone anywhere near this."
Asked whether the then-acting director general Tim Davie had signed off on the plans, Mr Thomas replied: "I can't be sure that Tim Davie did. I know that Tony Hall (the current director general) has been involved in recent days."
More top news
Noel Edmonds beat off competition from Ant and Dec to be voted Britain's favourite children's TV presenter.
A number of teenagers - including some who have never smoked - are experimenting with e-cigarettes, a study has found.
Singer Lil' Chris - who was found dead last week - suffered with depression and mental health issues for years, his family said.