Death by chocolate could become a reality after a labelling glitch left a crucial health warning off thousands of chocolate bars.

Trade Aid's chocolate bars may contain traces of nuts and/or dairy products which can cause anaphylaxis, a potentially fatal allergic reaction.

But a warning to this effect was left off a labelling print run at a Belgian chocolate factory Trade Aid uses.

As little as 1/2000th of a peanut can cause a lifethreatening reaction in some people, according to Allergy New Zealand.

"It's a huge concern, there could be people out there at risk," said educator Sarah-Jane Murison.

Anaphylaxis can be caused by a wide range of foods, including dairy products.

Mrs Murison said Trade Aid had acted responsibly.

It issued an allergy warning, and sent thousands of warning stickers to retailers, so bars which have not been sold can be relabelled, said Trade Aid New Zealand general manager Geoff White.

About 27,000 chocolate bars have been relabelled, and another 65,000 being shipped to New Zealand will also have to be relabelled.

The hitch was expected to cost Trade Aid $20,000.

Mr White said the company was unsure how many bars had been sold. "We are hoping that nothing does happen. We think it's very, very unlikely."

The lines affected are 50 gram milk chocolate bars, and 50 and 200g dark chocolate bars.

Bars missing the warning can be returned for a refund.