A website and Facebook post for Megabus, seen in September, for a new Aberystwyth to Birmingham service featured a yellow circle with the text "From Â£1", while another ad listed services from Leeds to new destinations in Yorkshire for the same price.
The page said Megabus was "offering a heap of new services from around the UK to many towns in Yorkshire", adding: "As always our fares are from Â£1*."
The Facebook post included text stating "Day trips from Â£1* London to Bath."
Two complainants, who searched for tickets and believed that fares were not available at the stated price, challenged whether the claim was misleading.
Megabus said that at the time of the website claim relating to the Aberystwyth to Birmingham route, there was one fare for that route for October 10 priced at Â£1 available online, while at the time of the Facebook ad there was one fare for the London to Bath route for September 22 available for Â£1.
Megabus said the number of fares available at Â£1 for any given route varied due to many factors, such as the number of intermediary stops and the number of tickets booked for the partial and full routes.
For the Aberystwyth to Birmingham route, which had five intermediary stops, there were a maximum number of 147 tickets for each departure, and theoretically six of those tickets were available at Â£1.
For the London to Bath route, which also had five intermediary stops, a maximum of 96 tickets could be sold for each departure, with three tickets theoretically available at Â£1, one of which was for the full journey from London to Bath.
Megabus said fares at Â£1 were generally available if the booking was made more than four weeks in advance, but this depended on the route.
The Advertising Standards Authority (ASA) said consumers would understand that not all tickets would be priced at Â£1, but would expect to be able to find the "from" price across a range of dates if they booked in advance.
Based on the numbers of Â£1 fares Megabus said were available on each route, the ASA understood that only 4% of fares for departures on the Aberystwyth to Birmingham route were available at Â£1, and 3% were available for departures on the London to Bath route.
"We considered that those figures did not represent a significant proportion of seats being available at the advertised 'from' price for those routes.
"We noted in particular that only one of the Â£1 fares on the London to Bath route was for the full length of the route," the ASA said.
It ruled that the ads must not appear again in their current form, adding: "We told Megabus to ensure in future that when using 'from Â£1' price claims a significant proportion of the advertised fares were available at Â£1, so that consumers would have a reasonable chance of obtaining the products at the advertised 'from' price."
A Megabus spokeswoman said: "We have always provided clear and transparent pricing for all of our customers, including the many thousands who have taken advantage of our great Â£1 seat offer.
"While we will continue to offer seats from Â£1, in line with recommendations from the ASA, we have decided to focus our marketing on the great value that all megabus.com fares offer, along with welcome extras such as on board wi-fi and charging points."