The Conservative party's election guru Lynton Crosby lobbied a minister against the introduction of plain packaging for cigarettes days before it was announced the London-based Australian lobbyist was to become campaign director for the Tories.

Crosby's London-based lobbying firm, Crosby Textor Fullbrook (CTF), was hired by Philip Morris International (PMI) in the autumn of 2012 because the owner of the Marlboro brand sought to derail plans to sell cigarettes in unbranded cartons, a move health campaigners claim will save lives by making smoking less appealing. Soon after winning the account Crosby made the case against plain packaging to Lord Marland, then parliamentary undersecretary for intellectual property and a former Conservative party treasurer.

On 18 November 2012, the Tories announced Crosby's appointment as their election strategist, giving the Australian lobbyist considerable influence. The following summer, when the government postponed plans to introduce plain packaging, critics were quick to accuse Crosby. But he insisted he had never abused his position, saying in July 2013: "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. Any claim that I have sought to improperly use my position as part-time campaign adviser to the Conservative party is simply false."

However, documents released by the Intellectual Property Office under the Freedom of Information Act show he did lobby Marland just before taking up his political role. In an email sent to Marland at 10:32am on 1 November 2012, carrying the subject line "IP [intellectual property] issues and Plain Packaging", Crosby wrote: "My dear Lord. Please find enclosed … a telling opinion from Lord Hoffman plus the view of Peter Lawrence (ex-Patent Division) and an interesting comment on counterfeiting from Crimestoppers.Hoffman is the most telling and concerning from an IP viewpoint."

Hoffman is a former lord of appeal and an expert in intellectual property law. PMI documents obtained by the Observer show that the tobacco giant intended to use intellectual property rights as a major weapon in its campaign against plain packaging.

Hoffman's opinion, commissioned by PMI, argues that "a prohibition on the use of a trademark, which has the effect of disadvantaging those with strong marks against those with weak marks", cannot be justified. Lawrence's article warned that UK "action to deprive brand owners of the right to use their marks would be an unfortunate precedent". The Crimestoppers briefing said plain packaging encourages counterfeiting.

Just over two hours later, Marland forwarded the email and the three attachments to civil servants in his department, saying: "Can we produce a reply please?" The civil servants then sent another internal email noting that the request had been "flagged".

Marland, who has described Crosby as "fantastic", is a close friend of the lobbyist. They worked together on London mayor Boris Johnson's campaigns. "When I was asked to get involved in Boris's campaign for London mayor, the first thing I did was call my friend, campaign guru Lynton Crosby, and ask him to run it," Marland once explained. CTF's website carries a testimony from Marland, calling Crosby "the best [campaign] manager in the world".

The email exchange, released to the Tobacco Control Research Group (TCRG) at the University of Bath as part of an ongoing investigation, raises questions about whether Marland discussed the issues forwarded by Crosby with anyone else in government.

"The documents reveal that Crosby was lobbying on the tobacco industry's behalf just days before it was announced he would be working for the Conservatives," said Anna Gilmore, professor of public health at the TCRG. "Both Crosby and David Cameron have serious questions to answer."

Deborah Arnott, chief executive of Action on Smoking and Health, said: "If the tobacco industry, the most profitable and least responsible industry on Earth, manages to block the plan to put cigarettes into plain packs, the prime minister will have a lot of explaining to do."

A spokesman for CTF Partners said: "Lynton Crosby was not working for the Conservative party in any capacity at the time of this discussion and there was no subsequent discussion with Lord Marland. There was no conflict of interest. The email contained documents that the government already had."

The government has yet to make a final decision on plain packaging.