Bell Canada has apologized for allowing a punk rock reference to the Holocaust to appear in one of its subway and bus shelter advertisements.

The ad promotes a brand of cellphone, but shows a teen dressed in punk rock style wearing a button that reads, "Belsen was a gas." It's the title of a controversial song by the 1970s band the Sex Pistols, which referred to the Nazi concentration camp Bergen-Belsen.

Bell said it pulled the ads as soon as it realized the problem.

"The button was very small, as you see in the ad, so during the approval process we couldn't actually read what was written on the button. In fact, it wasn't clear there were any words on the button. But once blown up to mural size it's easily readable," said Mark Langton, vice-president of media relations for Bell. 

It's estimated that about 50,000 people died in the concentration camp between 1940 and 1945, including Anne Frank and her sister Margot. When the camp was liberated in April 1945, 60,000 emaciated people were found in the camp along with thousands of unburied bodies. 

"It was an unfortunate situation, one we regret, and in fact an error on our part, and we took it down as soon as we could," Langton said.

The company has offered an apology to anyone who was offended.