David Cameron's most senior civil servant has rejected Labour calls for an inquiry into alleged breaches of the ministerial code by the Prime Minister as a result of his links with Conservative elections adviser Lynton Crosby.
Cabinet Secretary Sir Jeremy Heywood said that, as Mr Crosby was employed by the Conservative Party and does not "advise or lobby on Government business", he could see no purpose in an investigation.
In a letter to Labour leader Ed Miliband, Sir Jeremy revealed that the lobbyist had agreed "principles of engagement" with the party when he took up his advisory role, pledging "not to use access to ministers or special advisers to influence or lobby for changes in Government policy on behalf of his firm's clients".
Earlier, Mr Crosby - whose lobbying firm is reported to have worked for tobacco giant Philip Morris - denied that he had "any conversation or discussion" with Mr Cameron on the issue of plain packaging of cigarettes.
Any suggestion that he had used his position as an adviser improperly was "simply false", he said.
His comments came after a string of media stories raising questions about whether his advice to Mr Cameron was in any way influenced by his company's work for firms involved in the smoking and alcohol industry, private healthcare and shale gas "fracking".
Mr Miliband accused Mr Cameron last night of "bringing big tobacco to the heart of Downing Street" by hiring the Australian polling guru as an adviser shortly before shelving plans for standardised cigarette packages.
But in a statement issued by his company CTF Partners, Mr Crosby said: "The Prime Minister has repeatedly and clearly said that I have never lobbied him on anything, including on the issue of tobacco or plain packaging of cigarettes.
"What the PM said should be enough for any ordinary person but to avoid any doubt or speculation, let me be clear. At no time have I had 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.
"Indeed, any claim that I have sought to improperly use my position as part-time campaign adviser to the Conservative Party is simply false."