Welsh Water issued a vegan advert on social media, which suggested people should give up meat for a day.
The advert said: "Nearly one third of agricultural water use is related to animal products.
“Try going Meat Free for just one day!”
Company bosses deleted the advert after a flurry of responses by irate farmers.
NFU Cymru President John Davies said: “We are extremely annoyed that Waterwise has misinformed consumers that going ‘meat free’ will help save water when the facts being peddled, and endorsed by Ofwat, do not accurately reflect the real story of Wales’ water resources or water use in the production of Welsh food.
“We are bitterly disappointed that Dŵr Cymru Welsh Water initially put out social media posts that supported and promoted this message. While these posts have since been taken down, they did attract an adverse reaction from members of the public, many of whom are farmers who are rightly appalled at this unbalanced and unprovoked attack.
“Waterwise’s portrayal of livestock farming as a sector that is wasteful of such an important natural resource is entirely unfair and without foundation.
"Farmers should be left to do what they do best. Welsh farmers are champions of an industry that prides itself on its sustainable production methods, using Wales’ natural assets – an abundance of rainfall and the ability to grow grass – to produce safe, high quality, affordable and fully traceable protein that underpins the Welsh Food and Drink Industry, Wales’ biggest employer.”
Welsh Water has apologised for the tweet and issued a statement.
A spokesman said: "Water companies and organisations across the UK have been supporting this by sharing content, tips and offering water saving devices on social media this week.
“Monday’s theme highlighted the amount of water that can be involved in the production of meat, and suggested going meat-free for a day.
“The single post that we shared on Facebook received a number of comments – many from people who were unhappy with the post.
“We apologise if we have caused any offence and deleted the post from our social media channel. It is never our intention to cause upset to our customers.”