Clear Channel UK has held the contract since 2005 and maintains more than 30,000 bus shelters and bus stops in the capital. As part of the contract, Clear Channel sells advertising on 5,000 sites.
JCDecaux, which already operates street furniture for a number of London boroughs including Camden, as well as in Manchester and Glasgow, is expected to pitch for the business.
According to a tender document seen by Campaign, the contract is worth £200-500 million, excluding VAT, over an initial five years, with an option to extend the arrangement for an additional three. The tender is divided into eight lots and companies can bid for as many as they wish.
The reduction in the length of the contract from ten years and the division of the tender into lots follow guidelines set out by the Office of Fair Trading in 2012 after a competition investigation into the sector.
After this review, TfL will call a pitch for the £1 billion ten-year ad contract for the London Underground, held by Exterion Media.
TfL is then expected to review its contract to sell ads on the side of buses, which is also handled by Exterion.
Spokespeople at Clear Channel and JCDecaux declined to comment. TfL was unavailable for comment.