Nestl Rowntree bosses are investigating an incident in which thousands of Aero wrappers are understood to have had a rude message printed on them.

According to a source, the words "S**t bar" appeared where a Best Before date and code would normally be printed.

The company confirmed that a "misprint" had appeared on bars, but said that none of the offending chocolate had left the factory before the problem was spotted.

A spokeswoman declined to confirm what the words were, but said the company took the matter very seriously.

"We are investigating what took place last Friday," she said. "Until we know more details about what happened, we cannot say anything further."

She believed that about 20 pallets of Aeros had been affected.

The spokeswoman denied rumours that the entire staff on a particular shift had been threatened with the sack unless the culprit came forward by the end of the week. "We do not go in for threats," she said.

But she did not deny that the culprit faced being disciplined if he or she was found, stressing again how seriously the issue was viewed by the company.

The wrappers come to the factory on pre-printed rolls of paper, but the Best Before date and a code are printed on them during the production process in York. A union spokesman declined to comment on what had happened.

Updated: 10:40 Wednesday, March 31, 2004