Energy supplier E.On has said it is "incredibly sorry" for sending socks to customers in a bid to encourage them to turn the heating down.
It is the second major supplier to admit to a marketing gaffe when customers are facing a cost of living squeeze driven by energy prices.
Earlier in the week, Ovo's founder said he was "really embarrassed" about energy saving advice including "doing a few star jumps" and cuddling pets.
Energy bills are set to soar in April.
A new price cap looks set to add about £600 to the annual gas and electricity bill of a household with typical energy usage.
The socks were sent out by E.On Next - an arm of the energy giant which sells and promotes renewable electricity.
The footwear came with a message encouraging people to leave "lighter footprints" by turning heating down and lowering carbon emissions.
However, some customers reacted with anger and perplexity - especially when they were sent to elderly relatives. Now, the company has apologised.
Writing on Twitter, it said: "If you recently received a pair of socks from us, we would like to say we are incredibly sorry for how we have made some people feel.
"In light of the seriousness of current challenges that many people are facing, this mailing should have been stopped and we are sorry."
As first reported in the Daily Mail, the company said it sent the socks to 30,000 customers who responded to an energy saving campaign last year.
In a subsequent statement, a spokesman for the company said: "This activity was in no way designed to detract from the seriousness of the current energy crisis
"This campaign originally went ahead last year and was intended as a fun way to encourage people to think about 'lightening your carbon footprint' and isn't meant to be anything to do with the current challenges many people are facing."
He said E.On had invested billions of pounds into better insulation, efficient heating, and advice to customers.
E.On is the UK's second biggest energy supplier, after British Gas.
Stephen Fitzpatrick, the founder of the third-largest, Ovo, said earlier in the week that "someone had a bad day" when a link was sent to customers directing them to a blog encouraging them to save energy by hugging pets.
The latest gaffe comes on the day an alliance of charities called on the government for urgent action to tackle the energy bill crisis.
The 25 charities, including Save the Children, Age UK, End Fuel Poverty Coalition, WWF, Green Alliance and Greenpeace, said emergency funding was needed to support the most vulnerable.