The poster campaign encourages the public to invest in the cryptocurrency craze, appealing for them to look to Floki if they were not quick enough to buy into other popular cryptocurrencies, such as Doge, early enough.
Products such as Floki are known as meme coins because they are launched quickly with branding that responds to popular cultural phenomena.
In May, the ASA banned a campaign for the cryptocurrency exchange Luno that ran on the Transport for London (TfL) network.
The investigation, which was first reported by the Financial Times, comes days after Liberal Democrat and Green party members of the London assembly called on TfL to stop taking advertising from cryptocurrency-related firms until the UK government and Financial Conduct Authority set out new guidance on marketing.
TfL says that while it does not have a ban on the marketing of cryptocurrencies, the ad campaigns are more heavily scrutinised than those for other sectors. This includes adding a disclaimer on all ads making it clear that cryptocurrencies are unregulated in the UK.
Last year, the UK Treasury proposed the introduction of stricter rules for the financial advertising of most cryptocurrencies.
