Image source, Getty ImagesOvo Energy has been accused of "degrading" Welsh after axing services in the language.
The energy company said people could instead use online translation tools to read their bills.
Welsh language campaign group Cymdeithas yr Iaith called the move "insulting", while the Welsh language commissioner said she planned to contact the business.
Ovo said its Welsh services were only used by a small number of customers.
Welsh Language Commissioner Efa Gruffudd Jones told the BBC's Cymru Fyw: "We have previously conducted a standards investigation into energy companies and presented the report to Welsh government for consideration with regards to bringing these companies under the Welsh language standards."
The Welsh language standards were designed to ensure Welsh is not treated less favourably than English in Wales.
"I will now be contacting Ovo Energy asking to meet urgently to discuss the situation in the hope that they will be prepared to consider other options, while also reminding them of their duties to their customers within Wales."
Cymdeithas yr Iaith's Siân Howys said the suggestion that people should use online translation tools was "insulting".
She said: "A company like this, which makes a profit in Wales, should be operating bilingually."
"They're downgrading and degrading the status of the language," she added.
Last November, banking giant HSBC announced it was ending its Welsh language phonelines because they were underused.
Ms Howys said: "Cymdeithas yr Iaith is calling on Welsh government to ensure that there's legislation that forces companies like Ovo to operate bilingually in Wales.
"We're a bilingual nation, Welsh and English have equal status, and yet there's a company like (this) which is disregarding it."
Ovo Energy said it had notified the "small number" of customers who used its Welsh language customer service.
A spokesman said: "We'll be phasing out the removal of this service, with Welsh translated bills stopping from March, and dedicated Welsh support in place to help with any customer queries until the end of May.
"After this time, customers will have the option to access their bills through the app or request their bills in email format, to allow them to use online translation tools.
"We're very sorry for any inconvenience."
The Welsh government said it was "disappointed" by the news.
"Increasing the use of the Welsh language is a priority, and we'd like to see as many opportunities as possible for people to use the language daily," a spokesperson said.
"We're glad that the Welsh commissioner has contacted Ovo to discuss the matter and to remind them of their duty for customers in Wales."