Lion - the owner of the Pura and Dairy Farmers brands in Australia - has joined another milk giant in Parmalat and declared war on fast-growing New Zealand rival A2 Milk.
Australian consumers have warmed to A2 Milk, which contains only the A2 beta-casein protein.
Many people who cannot digest regular milk - which contains both the A1 and A2 proteins - say they can drink A2 without digestive issues.
The science surrounding A2 Milk has never been conclusive, but the word-of-mouth support from consumers has been overwhelming. It has won more than 5 per cent of Australia's fresh milk market from its rivals since launching in 2007.
To fight back, Lion relaunched the Pura brand in South Australia last month, with labels that say: ''Naturally contains A2 protein.''
According to Lion's website: ''We can now let South Australian consumers know that Pura milk naturally contains A2 protein, as well as A1 protein.''
Fairfax Media believes Lion is testing the new labelling before a relaunch in Victoria and NSW.
A2 Milk executives accused Lion of trying to confuse consumers. ''It's not like we add A2 protein to the milk, the fact is we exclude all [A1] so people with digestive issues get the benefit,'' said A2 Australia chief executive Peter Nathan.
''We are concerned that consumers may be confused and led to believe that Pura's product is somehow like A2 milk when it is not.
''Nothing has changed with Pura. It's the exact same milk they sold before the label change.''
Mr Nathan would not rule out legal action against Lion, or lodging a complaint with the Australian Competition and Consumer Commission.
''We are considering all our options, legal and otherwise, to protect consumers, our property and our trademark,'' said Mr Nathan.
A2 Milk has since launched a radio campaign in South Australia.
''You wouldn't want a cat that barks or a horse that moos,'' says the ad. ''So you wouldn't want A2 milk to contain A1 protein. Pura milk is like a barking cat.''
Last month Fairfax Media revealed Parmalat had engaged Crosby Textor - the spin doctors behind the 2013 election campaign of Prime Minister Tony Abbott - to try to discredit the science that underpins A2 Milk.
Crosby Textor was engaged late last year.
A spokesperson for Lion said: "The rollout of our new ‘naturally contains A2 protein’ on-pack messaging and marketing campaign responds to consumer interest."
"We stand by our on-pack claims and our testing methodology, which provides transparent facts to consumers on an issue they are interested in."