Tesla, the US electric car and battery maker, has lost the latest round of a long-running spat with UK energy company Ecotricity.

The company, owned by billionaire Elon Musk, had lodged a complaint with the UK’s Advertising Standards Authority (ASA) about claims on Ecotricity’s website that it supplies “Britain’s greenest energy” and “greenest electricity”. On Wednesday, the ASA dismissed the complaint - agreeing with Ecotricity that the claims are correct.

The advertising watchdog concluded in its ruling that only one other unnamed energy company – probably a reference to Good Energy – supplied electricity from 100% renewable sources, but on a lifecycle analysis of those renewable sources, Ecotricity produced a few grammes less CO2 per kilowatt hour generated.

On the basis that the Gloucestershire company’s gas supply was slightly lower emission than most because it used a small percentage of gas derived from sugar beet, the ASA said that Ecotricity’s claim to supply the greenest energy in Britain had been substantiated and was not misleading, and therefore the complaint was not upheld.

Dale Vince, Ecotricity’s founder and CEO, told the Guardian he was pleased the ASA had confirmed its claims: “Tesla’s complaints have never made any sense to us. Clearly they are unhappy with the way our recent legal proceedings ended, where we defended our Electric Highway from them.

“But our claims to provide the greenest electricity in Britain are very well documented and founded, so we were surprised they chose to pursue this — it’s been a bit of a waste of everybody’s time.”

The two companies have a bad-tempered history after a partnership between them turned sour. Ecotricity had teamed up with Tesla to help it build a network of charging points to support its customers’ high-end electric cars. But when the two companies fell out they reached a confidential out-of-court settlement in 2015.

Tesla has been increasing its foothold in the UK, most recently selling its Powerwall energy-storage system through third parties, as well as its luxury electric cars and building a car-charging network.

In 2014, Ecotricity accused Tesla of a “smash and grab”, saying it was trying to take over several sites on Ecotricity’s electric car-charging network at motorway stations across the country. The two companies had entered a partnership under a non-disclosure agreement (NDA) and Ecotricity alleged Tesla was asking motorway services operator Welcome Break to break off exclusivity contracts with Ecotricity and sign with Tesla instead.

The two companies countersued, ultimately leading to an out-of-court settlement last July. In a mutually agreed statement, they said: “Ecotricity and Tesla have reached an out-of-court settlement, the terms of which are confidential.”

A Tesla spokeswoman initially said the company had no comment to make on the ASA ruling, but later said: “Tesla and Ecotricity resolved their differences a long time ago, and we wish them the best.” The company has made headlines around the world in the last week with the unveiling of its more affordable Model 3 electric car, for which it has taken pre-orders worth almost £7bn in just two days, according to Musk.