Ryanair is under investigation for encouraging people to book their holidays as Covid vaccine is being administered.
The Advertising Standards Authority said: "We have received complaints about a Ryanair ad which encourages people to book holidays abroad now that vaccines are being administered.
"We have launched an investigation into whether the ad has broken our rules and will publish our findings in due course."
It comes as holiday firm TUI has said it is cancelling all trips from England up until mid-February after a third national lockdown was announced.
The travel operator cancelled all flights due to depart from England before mid-February on Tuesday as well as holidays from Scotland and Wales until 31 January.
The UK is currently in the middle of rolling out two vaccines.
Around 530,000 doses of the Oxford/AstraZeneca vaccine are available with vulnerable groups being given priority in the vaccination programme.
It is the second Covid-19 vaccine to be given the green light in the UK after the Pfizer/BioNTech jab began its rollout programme in December.
The vaccine has been developed in the UK by Oxford University and the pharmaceutical giant AstraZeneca.
England's Chief Medical Officer Chris Whitty warned vaccine shortages are likely to cause problems for "several months" with a lack of global supplies likely to hamper efforts to protect the UK in the first part of 2021.