BT's complaint addressed Virgin's national "Hate to Wait" advertising campaign, which used billboard, newspaper and television adverts to show how fast users could download songs and TV shows on a Virgin Media connection.
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BT complained that Virgin's usage caps meant that, during peak times between 4pm -9pm, users would not be able to download songs or TV shows at the speed and times cited in the ad.
After reviewing the case, the Advertising Standards Authority (ASA) agreed and has ordered Virgin to change its advertisements to state that speeds will vary.
Virgin media said in its advert that customers on its 'M' package (up to 2 Mb) could download a song in 22 seconds and a TV show in under 26 minutes.
But the ASA said the ad did not make clear that the 26 minute download time was only possible during off-peak hours, or that by downloading one full TV show in peak hours customers would automatically be in breach of the download limits for the M package.
"One of the main objectives of the ad was to highlight the speed with which customers could download a TV show. In the absence of any clarifying text, readers were likely to understand that those speeds applied at all times," said the ASA in a statement.
The ASA also considered that the text "acceptable usage policy applies" was unclear and it would not be unreasonable for readers to expect to be able to download at least one half-hour TV show on the M package.
Virgin said it had also confused the term 'megabits' with 'megabytes' when referring to the size of the files that could be downloaded and promised to use the correct term of megabytes in its advertising.
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