One person complained about the availability of the tickets at the reduced price, and questioned whether the ad was misleading.
National Express East Coast said all 80,000 plus seats were available to customers on a Standard Advance ticket up to one day before departure.
It said more than 10 per cent of the total number of tickets were available at the from price of £13 during the offer period.
The watchdog said the complainant believed the claim "So that should put a smile on 80,000 faces" implied there was a higher number of £13 one-way tickets available than there were.
But it said travellers could obtain considerable savings on tickets purchased with Standard Advance tickets on the day of travel, and that the poster made it clear that £13 was the from price of one-way tickets between York and London.
It felt consumers would understand that not all 80,000 tickets would be available at the from price and the range of low priced tickets available justified the ad’s claim.
It concluded the ad, by Leeds agency Smart, would not mislead and deemed no further action to be necessary.