The Office of Fair Trading says it has taken action against 13 airlines for leaving taxes, fuel fees and other charges off their advertised prices.
The OFT says some ads can distort flight competition
The OFT was concerned that website ads omitted some fixed, non-optional costs.
Eleven airlines have now changed ads and sites to include these costs in the prices they advertise.
Meanwhile, Aer Lingus and Ryanair have changed homepages - but say technical issues prevent them from changing their entire website booking immediately.
The OFT said it was satisfied the pair would be making the changes shortly.
In the interim Ryanair has agreed to place a clear statement on its website to inform consumers how much the taxes and charges will add to the final price.
Sean Williams, OFT executive director, said: "The OFT is committed to ensure that consumers are not misled by advertised prices that bear little relation to actual prices.
"Misleading pricing not only undermines consumer confidence but also distorts competition and we will enforce the law enthusiastically where we find that consumers have been misled."