David Cameron's decision to refer Sayeeda Warsi to the independent adviser on the ministerial code has prompted fresh outrage at his refusal to do so after Jeremy Hunt.
The prime minister asked Sir Alex Allan to investigate Baroness Warsi's conduct after it emerged she had failed to declare her connection with a business partner who accompanied her to Pakistan shortly after the general election.
Baroness Warsi has already apologised to the prime minister in writing, but insisted neither she nor Abid Hussain had benefited from the trip financially.
"I did not recognise, at the point that this visit was arranged, a need to disclose that Abid Hussain and I have a common business interest," she wrote.
"In retrospect, I accept I should have made officials aware of the business relationship between Mr Hussain and myself, and for this I am sorry."
Mr Hussain, the second cousin of Baroness Warsi's husband, was also a director of a company called Rupert's Recipes which the Tory party co-chairman had an interest in.
Conservative MP Louise Mensch defended Baroness Warsi on the Today programme, saying her mistake was "extremely minor".
But Mr Cameron's decision to refer the Tory party co-chairman to Sir Alex has sparked anger among opposition politicians, who were frustrated by the prime minister's refusal to make a similar move over his culture secretary.
No 10 instead insisted that the Leveson inquiry would provide sufficient scrutiny of Mr Hunt's conduct. The media secretary appeared before the probe into the culture, practice and ethics of the press last week, after which Mr Cameron declared himself satisfied with Mr Hunt's conduct.
"David Cameron's actions in this case draw into sharp relief his refusal to hold a similar investigation into Jeremy Hunt, despite clear evidence that he broke the ministerial code by misleading the House of Commons on at least three occasions," shadow Cabinet Office minister Michael Dugher said.
"David Cameron is bending over backwards to defend Jeremy Hunt because he knows that it is his own judgement, in appointing a man he knew to be biased to oversee the BSkyB bid, that is in question."