McDonald's new "eco-friendly" paper straws cannot be recycled and must be binned and burned instead, a report claims.
Last year the fast-food giant replaced plastic straws with paper ones across all of its UK and Ireland restaurants after a successful trial period.
But McDonald's said the new paper straws are not yet easy to recycle and should be put into general waste as a temporary measure.
McDonald's said the materials are recyclable, but the thickness of the straws make it difficult for them to be processed.
The firm said it was working to find a solution to the current advice, as first reported by The Sun, to put paper straws in general waste.
After criticism of the new straws, which some customers complained turned to mush in their drinks, a McDonald's spokesman said they were strengthened following feedback.
He said: "While the materials are recyclable, their current thickness makes it difficult for them to be processed by our waste solution providers, who also help us recycle our paper cups."
The spokesman added: "We are working with them to find a solution, and so the advice to put paper straws in general waste is therefore temporary.
"This waste from our restaurants does not go to landfill but is used to generate energy."
The fast food giant also said its plastic straws at the time were made from "100% recyclable material".
The move to paper straws came amid growing pressure on companies to reduce single-use plastic products and packaging.
It followed mounting concerns over plastic pollution in the oceans where items such as straws end up harming wildlife such as turtles and whales.
When it confirmed the move, McDonald's said the two suppliers it had found to meet its needs for paper straws were the start-up Transcend Packaging based in Wales, and global company Huhtamaki which would produce the straws at its plant in Belfast.