Plant baker Warburtons has been rapped over the knuckles by advertising authorities after complaints when it claimed it was: “No1. Now London’s biggest bakers”.

The complaint by rival bakery giants Premier Foods, maker of Hovis, and Allied Bakeries, the producer of Kingsmill, was upheld by the Advertising Standards Authority (ASA).

Warburtons said the claim - which was used in a national campaign - was verified by a global independent market research company, whose data for the week ending 25 May 2013 showed it had the largest market share over 52 and 12 weeks.

The ASA said the advert, which appeared in the national press, was misleading.

In a statement, the advertising watchdog said: “The ASA acknowledged that Warburtons had provided data from a global independent market research company, which showed they had the largest market share over a 52-week and 12-week period for the week ending 25 May 2013. However, we noted that Hovis provided data from their own independent market research sources, which contradicted Warburtons’ data.”

The ASA argued that the Warburtons data only included “grocery multiples and excluded impulse channels”, and it added: “We considered that a significant number of consumers were likely to purchase freshly baked products rather than, or as well as, wrapped bakery products. We considered that consumers would interpret the word ‘bakers’ to include a retailer that sold freshly baked products and would not interpret it to mean a retailer that exclusively offered wrapped bakery products.”

Warburtons was also criticised for not verifying the data source in its ad, although it had done so in other versions of the ad in other media. The ASA concluded: “We considered that consumers had not been provided with sufficient information to understand the basis of comparison and we therefore concluded that the claim was misleading.”

The advertising complaint highlights the bitter battleground in the nation’s supermarkets for bread listings. Premier Foods lost in the region of £25m when a contract to supply The Co-operative was lost in late 2012 to rival Allied.