The M&S 'Vitamin Water' drink is available in Orange & Passionfruit and Cranberry & Raspberry variants. It forms part of a product-development push by the retailer based around its 'food-on-the-move' section this summer.
As well as having a virtually identical name, the product is packaged in a similarly shaped 500ml bottle to Vitaminwater.
David Goudge, managing director at innovation agency Brand Development, said it was 'poor practice' on the part of the retailer. 'For a company with the imagination of M&S to end up doing something like that is just a shame,' he added.
Mary-Ellen Field, intellectual property director at brand valuation consultancy Brand Finance, said she believed M&S would argue that 'vitamin water' is a descriptive term, and therefore not an infringement of Coca-Cola's trademark.
'More than anything, it demonstrates it is a highly successful and innovative product category - imitation is the sincerest form of flattery,' she said.
Coca-Cola acquired the Vitaminwater brand in 2007 for £2.1bn and introduced it to the UK in 2008. Last month, the brand was backed by its biggest UK marketing push to date, spanning outdoor and digital.
Sales of functional water are on the rise. Last year, the category recorded growth of 72%, with Glaceau Vitaminwater increasing sales by nearly £5m; PepsiCo's V Water, rose 143% to hit annual sales of more than £1m, according to the Britvic Soft Drinks Report 2010.
A Coca-Cola spokeswoman said: 'We do not comment on competitor activity.' M&S was unavailable for comment as Marketing went to press.