Michael Gove drank from a disposable cup in an appearance before MPs while pledging to cut his use of single-use plastics for Lent.
The Environment Secretary used the biodegradable cup during the Environment, Food and Rural Affairs Committee hearing on Ash Wednesday as he said he would "seek to minimise" his plastic use in the run up to Easter.
"I said that I would reduce my use of single-use plastic," he told the MPs.
Compostable, plant-based cups were introduced to the Houses of Parliament last year to replace existing single-use plastic items.
The cup Mr Gove used was made from polylactic acid, PLA, a renewable alternative to plastic.
But environmental campaigners say reusable cups are still better for the planet and critics point out that many products that are labelled biodegradable cannot just be thrown onto a compost heap with food waste.