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:
Cabinet Secretary Sir Jeremy Heywood told Labour he does "not see what purpose would be served" by an inquiry into claims that Tory adviser Lynton Crosby lobbied David Cameron over tobacco packaging.
In a letter to Labour leader Ed Miliband, who has demanded an inquiry, Sir Jeremy said:
Cabinet Secretary Sir Jeremy Heywood has rejected Labour calls for an inquiry into alleged breaches of the ministerial code relating to lobbyist Lynton Crosby's engagement as elections adviser to the Conservative Party.
Labour vice-chairman Michael Dugher said Lynton Crosby's statement denying he lobbied the Government on tobacco issues "raises more questions than it answers":
Lynton Crosby, whose lobbying firm is reported to have worked for tobacco giant Philip Morris, said any suggestion he had used his position as an adviser improperly was "simply false".
In a statement issued today by his company CTF Partners, Crosby said:
Conservative election adviser Lynton Crosby has denied having "any conversation or discussion with or lobbied the Prime Minister, or indeed the Health Secretary or the health minister, on plain packaging or tobacco issues".
The Prime Minister will not be able to move on from the Lobbying row until he answers questions over Lynton Crosby's influence on government policy, according to a public relations expert.
Kevin Craig, from Political Lobbying and Media Relations, said Lyton Crosby's involvement in government policy "looks wrong and and doesn't feel right" and called on the Prime Minister to provide answers.
I asked the Prime Minister if Lynton Crosby advises him that voters don't like politicians serially evading questions.
Response: I dealt with that yesterday.
The NHS links pre-dated David Cameron hiring Mr Crosby however it is the second time the Prime Minister has had to defend his controversial election strategist over claims of lobbying.
Mr Cameron has insisted that Mr Crosby had not intervened in policy decisions, but has repeatedly refused to answer questions about the extent of his conversations with the lobbyist.
The Prime Minister has come under sustained pressure to say whether Mr Crosby, whose lobbying company has also worked for tobacco giant Philip Morris, had spoken to him about shelving the plan for cigarettes to be sold in plain packaging.