Vodafone has been forbidden from re-running an advert for its home broadband service after it was challenged by rival Virgin Media.
The complaint centred on Vodafone's branding of a premium offering targeted at gamers and other heavy internet users as being Gigafast Broadband.
Virgin Media said that term implied that its competitor was providing speeds of one gigabit per second.
However, Vodafone's own ad recognised this was not typically the case.
It stated that the maximum average speed available was 900 megabits per second.
This was in line with rules introduced in May 2018 that restrict internet providers to only advertising download speeds that are available to at least 50% of their customers during the peak hours of 20:00 to 22:00.
When the matter was raised with the Advertising Standards Authority, the watchdog said it thought that most consumers would consider the prefix giga- to be a "hyperbolic description of speed". However, it added that a significant minority could still be fooled into thinking the term referred to downloads of 1,000Mbps and higher.
The ASA considered the term might still be justified by the fact the service was capable of achieving speeds of 1Gbps outside of peak hours and/or to a minority of subscribers.
But it added that this would still not excuse the line "enjoy Vodafone Gigafast Broadband speeds for as little as £23 a month", since the lowest-priced package topped out significantly slower.
"We considered that the... claim, unmoored from reference to a specific package, created the impression that a service that could achieve speeds of 1Gbps was available for £23," the ASA said.
"In fact only the average 100Mbps package could be purchased for £23, while the average 900Mbps package cost £48 a month.
"We concluded that it was likely to mislead."
This marks the third time since September that a complaint about Vodafone's adverts has been upheld by the regulator.
Vodafone has since amended its website to say: "Our packages start at £28 per month for new customers purchasing Gigafast Broadband 100."
A spokeswoman defended the decision to retain the Gigafast branding saying: "We had made improvements to our website several months before the ASA ruling to ensure that the cost and speed of each package is as clear as possible."