At least 55 million single-use cups have been bought by public bodies in Wales over the past five years, costing about £1.7m, new figures have shown.
Hospitals, councils and government organisations have said they were trying to reduce a throwaway culture.
But data showed the number of disposable cups bought increased by about 10% between 2013 and 2018.
Twenty-seven out of 57 public sector organisations in Wales, such as hospitals and councils, told BBC Wales how many single-use paper and plastic cups they had bought over the previous five years.
It is estimated about 600 million throwaway cups in total were used in Wales over the same period.
The Welsh assembly told BBC Wales it saw a 20% rise over the past five years in the number of disposable cups it bought.
A spokesman said it would switch away from single-use plastic cups in September and use compostable and recyclable cups instead.
Responding to the data, David Melding AM – the Shadow Secretary for the Environment – said:
“These are some startling statistics, but we should be encouraged by the significant public support that now exists to reduce the use of these materials in Wales.
“As far as I’m concerned the Welsh Government and the Assembly should be leading this fight, and so I am glad that the Assembly reacted to my campaign to undertake Plastic Free July here in Cardiff Bay.
“The beauty of the Plastic Free July initiative is that it shows people just how easy the transition away from the use of plastic materials can be, and I’d ask everyone in Wales to commit to such a period of time as an initial step to ending this plastic scourge.
“Wales could be leading this global campaign, and what a fantastic year 2018 is to start – Wales’ Year of the Sea”.