Chancellor George Osborne, who is also the Conservatives' election chief, refused to reveal when exactly an agreement was reached to ensure the party's election adviser Lynton Crosby did not use his role to influence ministers and aides on behalf of his lobbying firms.
The Prime Minister had come under sustained pressure to say whether Mr Crosby, whose lobbying company has worked for tobacco giant Philip Morris, had spoken to him about shelving the plan for cigarettes to be sold in plain packaging.
On Tuesday, Mr Crosby denied the claims as "simply false." The Cabinet Secretary has also dismissed Labour calls for an inquiry.
But Mr Osborne refused to answer repeated questions from Economics Editor Richard Edgar over when the "principles of engagement" document was agreed: