Geoffrey Keeys, a non-executive director and a director of the Prudential insurance company, announced yesterday that he was resigning after the Home Secretary had failed to respond to his backing of Mr Lewis. A second board member, Millie Banerjee, a BT director, said she was considering resignation and would make a final decision by tomorrow.
Letters from Sir Duncan - seen by the Independent - show that the full extent of outrage on the Prison Board about the report into security by General Sir John Learmont, and its implications for Mr Lewis's future, was well known to the Home Office before Mr Howard took the decision to sack him.
Sir Duncan's warning that "change would be severely damaging to the future of the service" and he "shuddered" at the consequences, is especially significant because of the high regard that Sir Duncan, a former chief executive of the NHS, is held in by ministers.
The first letter on 11 October says that if Mr Howard was contemplating any changes at the top, the non-executive board members would want to meet him "before any decision was taken". But in his second letter, on 13 October - presumably after a negative reply from the Permanent Secretary - Sir Duncan complains that "it is very regrettable that despite the extended timetable of the Learmont review it may not be possible for non-executives to make their representations direct to the Secretary of State".
Complaining that the Learmont report is a "serious distortion" Sir Duncan says that a letter he himself wrote was "misquoted selectively - without the critical introduction".
This latest blow to Mr Howard's authority came after a tense seven-hour meeting of the Prison Board yesterday.
The board, which currently comprises the Prison Service's director-general, five full-time directors, and four non-executive directors, makes day- to-day operational decisions. It monitors the Prison Service and makes recommendations to the Home Secretary on policy issues. Mr Howard appoints all the members to the board.
Mr Keeys said in his resignation letter to Richard Tilt, acting director- general of the Prison Service: "As you are aware, I believe strongly that it was in the best interest of the service that Derek Lewis remain as director-general.
"I am assured that opinion was communicated to the Home Secretary but it was clearly one he did not share."
A third non-executive board member, Bill Bentley, a former manager at Shell, said he would not be resigning but also expressed support for Mr Lewis.