Sainsbury's released the drink Pitchers in April this year, advising shoppers that it could be mixed with lemonade and fruit -- which is how Pimm's No. 1 is traditionally served.
A comparison of the packaging reveals striking similarities. The bottles are a similar shape, both with red lettering in a similar sans serif font, with the logo topped by a gold crown, in the case of Pimm's, and a gold lion for Pitchers.
Sainsbury's promotion of Pitchers in no way shies from Pimm's positioning, advising that it has been made available "in time for all the top summer events -- Ascot, Henley and of course, Wimbledon".
Diageo has spent millions of pounds advertising Pimm's over the last few years in ads playing up to its image as a bit of a toff's drink, using the strapline "it's Pimm's o'clock".
Sainsbury's publicity for Pitchers also highlights that it is cheaper than its branded equivalent, as well as performing better in taste tests.
The dispute is a gamble for Diageo because Sainbury's is one of the country's biggest retailers and one of their major customers.
However, Diageo has the muscle of a wide drinks brands portfolio including Guinness, Johnnie Walker, and Smirnoff.
Diageo has confirmed that it is taking legal proceedings in relation to an intellectual property matter but said: "It is Diageo's firm intent that our strong trading relationship should not be affected by this discreet dispute."
Sainsbury's has responded by saying it will defend itself "vigorously" against the claims.
In 1997 a 'copycat' case involving United Biscuits and Asda reached court. Asda was found guilty of passing off its own-brand Puffin bar as United Biscuits' Penguin chocolate biscuit. The supermarket was permitted to retain the name as long as it amended its packaging.