A bar at the Selfridges department store has been forced to increase servings of wine from a dispenser.
Selfridges said the rules were "outdated"
Trading Standards said small quantities of 25ml and 75ml measures, which were being served in the Wonder Bar in central London, were illegal.
The self-serving wine "jukebox" is now dispensing 125ml and 175ml amounts as required by law.
Selfridges said the rules were "outdated" and that wine tasting is done in "small sips".
'Not kept pace'
Sue Jones, chief trading standards officer at Westminster City Council, said: "Trading Standards officers have recently advised Selfridges department store about selling wine in illegal amounts.
"Current legislation states that wine must be sold in measurements of only 125, 175ml and multiples thereof."
Selfridges' food and restaurants director Ewan Venters said: "We certainly didn't set out to break the law, but it seems regulation has not kept pace with technology or with what people wish to learn about wine by tasting small sips."
The wine jukebox contains 52 types of wines and vintages which customers serve for themselves. Prices range from £3.45 to £160 for a 125ml measure.