McDonald's has admitted that its paper straws are not recyclable, while the plastic ones they replaced were.
The "eco-friendly" paper straws were rolled out across 1,361 McDonald's restaurants in the United Kingdom and Ireland last year in a bid to "protect the environment" following a successful trial.
However, the fast-food giant said on Monday that the new straws were too thick to be processed by their recycling partners.
"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," a spokesman for the company told the Press Association news agency.
The straws had been thinner originally, but were strengthened after customers claimed that they disintegrated before a drink could be finished and that it was difficult to drink thicker liquids, such as milkshakes, through the paper straws, the spokesman said.
A leaked internal memo, published by the UK's The Sun newspaper on Sunday, said the new straws would be thrown away and burned, rather than recycled.
"Paper straws are not yet recyclable and should be disposed of in general waste until further notice," the memo said.
However, the McDonald's spokesman said on Monday that the company was working "to find a solution so putting paper straws in the general waste is therefore temporary".
"The waste from our restaurants does not go to landfill, but is used to generate energy," the company added.
Before the switch to paper, McDonald's said it used 1.8m single-use plastic straws each day in the UK. The decision to use paper straws followed a proposal by the UK government in April 2018, pledging to ban plastic straws and cotton buds entirely by 2020.
While the company was initially praised for the swap by many, including the UK's then-Environment Secretary Michael Gove, it has now faced fresh accusations of "greenwashing" by activists who say small changes to packaging are dwarfed by the enormous effects meat production had on the environment.
A petition to have the recyclable plastic straws reinstated has gained more than 51,550 signatures since it was created.