Britain's most senior civil servant was drawn into the row over the Tories' chief election strategist Lynton Crosby on Tuesday after he rejected a call by Ed Miliband for an inquiry on the basis of a Conservative document drawn up in recent days.
Labour, which claimed that the "mystifying" document appeared to have been "stitched up" at the last minute, is expected to write to Sir Jeremy Heywood on Wednesday.
The row erupted hours after Crosby moved to close questions about his role in influencing government policy by issuing a denial that he had ever discussed or lobbied David Cameron or the health secretary over tobacco packaging. The move follows months of prevarication by the prime minister with carefully worded denials.
The Crosby statement was followed by a letter from Heywood to Milibandin which the cabinet secretary rejected the Labour leader's call for an inquiry into the Tory election strategist on the grounds that the party had drawn up "principles of engagement" with him. "Against this background I do not see what purpose would be served by the enquiry that you propose," Heywood wrote to Miliband.
The "principles of engagement" are undated and offer no suggestion of when they were drawn up. A Conservative spokesman admitted that the principles had been written in recent days, though he insisted they reflected the terms of Crosby's verbal agreement with the party when he started to advise Cameron last November.
A spokesman said: "The principles of engagement capture what was agreed when Lynton was hired, and has been adhered to since. Verbal agreement on the principles of engagement was made at that time. This was written down in last couple of days and published today."
Labour is to write to Heywood about his decision to reject an inquiry into Crosby's role on the basis of a Tory document drawn up in recent days after Downing Street faced questions about its strategist's business interests.
A senior Labour source said: "This is a mystifying document. It appears to have been stitched together at the last minute with no date and no details.
"It has clearly been drafted in a few hours and appears not to be part of a legal document. We will be seeking further clarification."
Crosby has faced questions over his business interests after it emerged that his company advises the tobacco group Philip Morris International. Cameron recently announced that the government would be shelving plans to introduce plain cigarette packaging, in the face of strong lobbying by the tobacco industry.
The document released by Heywood says that in order to avoid a conflict of interest between Cameron's role as prime minister and Crosby's business interests a series of principles were agreed.
These are:
• "Lynton Crosby would not use his CCHQ position, or access to ministers, to further his own private interests or the private interests of others.
• "He would not use access to ministers or special advisers to influence or lobby for changes in government policy on behalf of his firm's clients.
• "He would not help – or give the impression of helping – a client by claiming to have privileged access to ministers, special advisers or officials."
The Cabinet Office issued a carefully worded statement in response to questions about when the document was drawn up. A spokeswoman said: "The principles of engagement capture what was agreed between the Conservative party and Lynton Crosby at the time that he was hired. Jeremy included them in his letter for reference only."
In his letter to Miliband, Heywood explained that he was rejecting his call for an inquiry in light of the agreement between the Tories and Crosby. Heywood endorses the Tory position that the terms reflect an eight month agreement between the party and Crosby over his business interests even though it was only codified in recent days.
The cabinet secretary wrote: "I attach the principles of engagement between Lynton Crosby and the Conservative party that have been the practice since Lynton Crosby was contracted by them. I hope this will reassure you about the processes we have."
In addition to asking Heywood about the document, Labour is also expected to ask for details of how often Crosby has attended meetings in Downing Street and Chequers. Labour is suspicious of the final sentence in the document which says that "no request was made for him to have access to government facilities" – and therefore no pass to Downing Street has been issued – on the grounds that he advises the Conservative party and not the government. Labour believes this is likely to have cleared the way for Crosby to visit Downing Street and Chequers on ad hoc basis.
The row came after Crosby insisted that he had never had any conversation with the prime minister or the health secretary about tobacco. In a statement issued by his firm CTF Partners, Crosby said: "At no time have I had any conversation 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 campaign adviser to the Conservative party is simply false."