He said: "The tower will add something to, not detract from, St Paul's. I still don't understand what the problem is."
Piano said English Heritage's method of analysing views had "taken things out of context" and was "perverse". He added the scheme was "conceived keeping in mind the history of the city".
He added that developer Sellar Property was not building for the sake of it. "Towers are usually about making money and the challenge of being tall. This is an intelligent sustainable building, not arrogant or stupidly symbolic."