TfL has responded to criticism of adverts by companies which appear to promote lets for longer periods than allowed under the Deregulation Act 2015.
The Act made it illegal for landlords to rent out their homes in the capital for more than 90 nights a year on short-term lets unless they obtain planning permission.
The new guidance says: "Indications of possibly problematic adverts include references to holiday lets which do not appear to be limited in time, ambiguity about the type of letting, and suggestions of increased returns from non-standard lettings."
However it added that these are simply indications, and the full content of any advert and the context of any words or images will be taken into account.
Assessment of adverts will take into account whether adverts imply that removing properties from the private rental market for short-term lets can be financially advantageous.
In addition, TfL will look at whether adverts promote "companies or services which appear to rely on property owners removing their properties from the private rented sector".
According to the guidelines, phrases which could be considered inappropriate in adverts for short-term lets include: "yield", "return on investment", "portfolio", "rental mix", "commission", "landlord", "tenant" and "letting agent".
According to the London Assembly Labour group, a recent investigation by the BBC found that short-let holiday rental firm Hostmaker, alongside a number of other short-term lettings management companies, was offering services to landlords to help them evade the 90-day limit.
London mayor Sadiq Khan instituted a review of TfL’s advertising guidelines in June after Hostmaker launched two new campaigns on the London Underground.
A week prior to that decision, Hostmaker said it had pulled the campaign and said it acknowledged the tone was "misguided".
Mayor of London, Sadiq Khan, said: "When everyone sticks to the rules, short-term lettings can help people earn a little extra money and they provide more options for visitors. But it is clearly wrong when companies encourage hosts to break the rules, and when services seem to rely on homes being taken out of the private rented sector altogether for financial gain.
"That’s why I welcome TfL issuing guidelines to ban adverts that encourage unlawful short-term letting and that make it harder for Londoners looking for somewhere to rent."
Labour’s London Assembly housing spokesperson, Tom Copley said: "It is hugely positive to see TfL take action towards these adverts on the back of concerns that I previously raised along with [campaign group] Generation Rent.
"However, it should be stressed that this move marks just one of the ways that we can strengthen regulation in the ever-growing home sharing sector to defend our long-term rented housing stock."
In June, plans for the redevelopment and extension of parts of the prestigious Dolphin Square residential development in central London to include 230 new apartments were refused, against a recommendation of approval from planners, partly on grounds that the scheme would provide more unwanted short term let properties.
In April, Westminster City Council called on the government to introduce tougher legislation to help it stop illegal online lettings, after an investigation by the local authority discovered a ten-flat residential building operating as a "virtual hotel".