Parliament's standards watchdog says it might look again at the ministerial code following a complaint about a paid lecture Cherie Blair made in America.
Mrs Blair gave a speech in Washington about life at Number 10
The Tories say it went against the spirit of the code which prevents government members making money from activities relating to their job.
The committee has told Tory Chris Grayling that it has "no mandate to investigate individual cases".
And it is constrained to look only at the conduct of public office holders.
However, a letter to shadow Commons leader Mr Grayling says the committee is about to start another inquiry.
"It is clear that there is genuine interest in the content and operation of the ministerial code to carry it forward as one possible and important inquiry subject," the letter added.
The Tory complaint followed Mrs Blair's decision to give a talk in June at Washington's Kennedy Centre - billed as the inside account of the "First Lady of Downing Street".
Commenting on the letter Chris Grayling said: "It was always quite clear to me that Mr and Mrs Blair did not break the ministerial code, they were certainly acting in breach of the spirit of the code.
"The committee does not have the power to investigate Mrs Blair specifically, but I hope that its response to my request is a clear indication that it will go ahead with a review and recommend tighter rules for the future.
"It is also completely unacceptable that the government has ignored a sensible recommendation for an independent adviser on the ministerial code who might have been able to prevent what turned into an extremely embarrassing episode."