Newsnight's editor should resign over a "patronising" response to complaints about the programme's item on the Welsh language, campaigners have said.
Ian Katz said some of the reaction to the debate on whether the language was a "help or hindrance to the nation" had a "whiff" of "unwillingness" to tackle questions over its promotion.
Cymdeithas yr Iaith Gymraeg called his letter "belittling".
The BBC previously said the item should have included a Welsh speaker.
It said it had no new statement to make on Mr Katz's letter, which also said Cymdeithas was unable or unwilling to appear on the programme - a claim the organisation said was untrue.
The letter was responding to North Wales Police and Crime Commissioner and Plaid Cymru member Arfon Jones, who complained to Mr Katz following the programme.
Mr Jones wrote his letter in Welsh and in an initial response Mr Katz said: "Did you mean to send it to me in Welsh? If so, you'll appreciate that I won't be able to reply till I have had it translated."
In a further letter to Mr Jones, the editor accepted the programme's guest casting was not good enough, and the wording of its introduction "was cruder than it should have been".
But he said he would "strongly argue that the question of whether the public promotion of the Welsh language is effective and beneficial to Wales is a perfectly legitimate subject of debate".
"We should have approached it with more subtlety, I agree, but there is a whiff in some of the response to our item of an unwillingness to even countenance such an impertinent question," he added.
Mr Katz also said it was a "fair" point the debate should have included a fluent Welsh speaker, adding the Welsh Language Commissioner and Cymdeithas were invited on "but they were sadly unable or unwilling to participate".
Cymdeithas said that was untrue and they had told a researcher the group could appear live and had offered to go to a studio.
The Welsh Language Commissioner said it informed Newsnight the programme could use an interview which had already been given to the BBC, and also provided the programme with a statement.
The organisation's chairwoman Heledd Gwyndaf said: "This is part of a pattern by the BBC of belittling Wales and the Welsh Language.
"The letter is factually wrong, arrogant and patronising."
She added that the letter was so "frightening" she believed "the editor of Newsnight should resign due to his prejudicial attitude".
Mr Jones called the response "trivial and soulless" and "patronising in tone".
He added: "They have tried to apologise but it's not really an apology."
More than 8,000 people have signed a petition calling for an independent review of how the BBC portrays the Welsh language.
In a statement issued earlier in August the BBC said: "Whilst different perspectives were included in this item on the Welsh language, the discussion of such an important subject would have benefited from more thorough analysis and debate.
"We regret that, but believe it was important to look at this topic and we will do so again in the future."
A later statement said it understood why viewers felt the item was "biased" with guests "poorly chosen".
A senior editor from Newsnight will be in Wales next month on a fact finding mission visit arranged by the Welsh Language Commissioner.