The company had boasted there were “plenty of tickets” for Newcastle-Edinburgh journeys available at £6.90, but rail fares campaigner George Eckton said he could find only one fare at that price over a three-month booking period.
The advert stated: “LumoFixed fares start at £6.90. With plenty of seats for this price when you book 12 weeks in advance.”
It made similar claims for other tickets, such as its £19.90 Edinburgh-London fares.
The Advertising Standards Authority (ASA) said the wording was “likely to have breached the advertising codes” and it had taken steps to address it.
Lumo said it had accepted the ASA’s guidance and changed the advertising on its website.
The firm, run by Aberdeen-based FirstGroup, launched services on the east coast main line in October, which have since doubled to four a day.
It is seeking to compete with LNER, which operates services every half hour, by offering lower fares if booked in advance.
The move comes as another victory for Mr Eckton after ScotRail agreed to change a “potentially misleading” advert about its fares after he raised it with the ASA.
Mr Eckton, a former director of the South East of Scotland Transport Partnership co-ordinating body, said: “Railway fares should be as cheap as possible, but marketing needs to be decent and not misleading.
"I’m glad the ASA has again issued advice to train operators, and specifically in this case Lumo, on how to change their advertising and website to give a fairer impression of the amount of fares available at the lowest prices each day.”
The ASA told him: “You were challenging the use of the claim ‘plenty of fares’ when you were unable to find more than one discounted fare in a 12-week advance booking period.
"We’ve assessed the ad you highlighted and, from the information we have, we think it likely to have breached the Advertising Codes (rules, 3.1, 3.7 and 3.17) that we administer.
"We have decided to resolve your complaint through the provision of advice.
"Therefore, we have explained your concerns to the advertiser and provided them with guidance on how to ensure that their advertising complies with the codes.”
A spokesperson for Lumo said: "Thank you for getting in touch about this matter and bringing it to our attention.
“We have taken guidance from the copy advice team at ASA and amended the website copy accordingly.
"Lumo will also be working with the ASA copy advice team to make sure that all of its web copy and adverts comply with all relevant guidance.”
In 2019, the ASA asked ScotRail to change adverts that implied the cheapest tickets could be found on its website.
It followed Mr Eckton pointing out passengers could get lower fares on some routes by taking advice from ScotRail staff at station booking offices on how to “split” tickets – buying separate tickets for different sections of their journey.
These include for journeys between the Glasgow area – where fare increases were historically kept lower than the rest of Scotland – and other parts of the country.