A Canadian political party with environmental policies has admitted altering a promotional photo to show their leader holding a reusable cup.

The picture, a candid shot of politician Elizabeth May talking to constituents, appeared on the Green Party's official website. 

In the original shot, taken earlier this year, May could be seen clutching a disposable coffee cup. But the website image shows her with a reusable eco-friendly cup, complete with the party's logo and a metal straw.

Some Canadian media outlets had used a cropped version of the original image, which didn't include the cup but did make it obvious there was no straw. When journalists pointed out the difference between the two photos, the Green Party admitted it had been deliberately edited.

A spokesperson told The National Post a decision was made to Photoshop a more 'on-brand' item into May's hands so as to display the party logo - not to communicate May's virtuous environmental values.

One of the Green Party's policies is a complete ban on single-use plastic items.

May herself has blamed the embarrassing incident - less than a month out from Canada's federal election - on "well-meaning party staff".

Ironically, the cup she was originally photographed holding was biodegradable.