Mobile portal publishers including Vodafone and ITV are scrambling to create new inventory in a bid to meet demand from advertisers.
Site owners have reported their inventory is sold out as advertisers and agencies embrace mobile.
ScreenTonic, which manages ad inventory on the Orange portal, has seen huge take-up from agencies. Its network of premium sites has been sold out for the last three months.
In response, Vodafone Live is to create more inventory. It has highlighted automotive and technology as key areas for advertising and will launch dedicated mobile channels. It's understood to have line up content companies like CNet, Dennis and Future to contribute.
The operator has previously secured ad deals in the sector with EA, Ford, Nissan, RAC and Sony.
Julie Jeancolas, associate director of Carat Digital, said mobile ad space was in short supply: "We're always keen to get more inventory."
Al Russell, head of mobile internet and content services for Vodafone, said "Advertising follows footfall and must be relevant in the right categories. Automotive and technology are good ad-led opportunities."
ITV has also launched a mobile ad campaign to boost its traffic and inventory, and is opening new on-portal opportunities for ads.
The broadcaster is on the hunt for its first mobile advertising sales manager to sell sponsorship, pre-rolls and interstitials around its programmes on mobile, like this week's Secret Diary of a Call Girl.
Melissa Goodwin, head of mobile at ITV, said, "It's a new space and not something we currently have the skills to sell."
The reaction from publishers mirrors the shift that took place online last year when premium portal owners like AOL, Google, MSN and Yahoo! introduced more advertorial and tenancies.