Leaked documents suggest a group supported by tobacco companies will intensify its campaign against plain cigarette packaging this weekend.

The Alliance of Australian Retailers has been running advertisements claiming small business owners will find it hard to distinguish different brands if plain packages are introduced.

Internal emails, invoices and contracts obtained by ABC1's Lateline show three tobacco companies paid more than $5 million to the alliance.

They also show plans to spend a further $4 million to target viewers watching the AFL and rugby league finals from this weekend.

In May, before the birth of the Alliance of Retailers, Philip Morris sought advice from a lobbying and public relations firm called the Civic Group about how to fight the Government's plan to introduce plain packaging for cigarettes by 2012.

The Civic Group's proposal recommended using third parties, including retail groups, to sell the message.

The Alliance of Australian Retailers was then formed in August, purporting to represent thousands of ordinary shopkeepers.

On the day the alliance was set up, documents show it became the instant beneficiary of millions of dollars from the world's top cigarette manufacturers: Philip Morris - $2.1 million; Imperial Tobacco Australia - $1.08 million; and British American Tobacco -$2.2 million

Curtin University professor Mike Daube says tobacco companies are desperate to fight the Government's proposal and the alliance's campaign is not what it seems.

"We see the spend of around $5 million on the first phase and now there's another $4 million going into the next phase," he said.

"I think this shows us how desperate the tobacco industry is. They're willing to throw millions and millions, as much as it takes, into opposing plain packaging."

Professor Daube, who was one of the medical experts who recommended plain packaging laws, believes the documents are possibly the most devastating tobacco industry leak ever seen in Australia.

"What this tells us is that far from being the innocent retailers' campaign that it pretends to be, this is a campaign that has been developed, masterminded and run to the finest detail by the Philip Morris company," he said.

"Now this is an international company, and it becomes so clear from these documents that it's been run by the Philip Morris company in association with British American Tobacco and Imperial."

In a statement, Philip Morris said there is nothing new about the fact that it supports the efforts of the Alliance of Australian Retailers, urging the Government not to proceed with the proposal for plain packaging.

"What is disappointing and concerning is that the documents we understand tonight's program will refer to seem to have been obtained by illegitimate means," the statement said.

British American Tobacco also said in a statement that it continues to support the Alliance of Australian Retailers in their campaign.