Britain's advertising regulator on Wednesday banned two adverts by Spanish clothing group Zara for featuring models that appeared "unhealthily thin," calling the images "irresponsible."

The Advertising Standards Authority (ASA) said it took action after it received a complaint about the ads, which were listed on Zara's website in May.

One image showed a model with "protruding" collarbones, with her pose and styling making her appear "very slim."

Another featured a model who looked "slightly gaunt" owing to a slicked-back hairstyle and that the lighting and clothing made her appear "noticeably thin," the ASA said.

The watchdog ruled the ads breached social responsibility rules and must not appear again in the same form.

Zara told the ASA that the models were medically certified as healthy, in line with British guidelines.

It also assured that only minor lighting and colouring edits were made on the images.

The ads were removed after the ASA made the company aware of the complaint, Zara said in a statement.

It added that Zara "follow stringent guidelines and controls in the selection and photographing of models."

Earlier this year, the ASA banned similar ads from British retailers Next and Marks & Spencer.