Ofcom said 'lots' making up a total of 150MHz of spectrum within the 3.4 GHz band will be sold during the auction.
A further 40MHz of spectrum will also be auctioned off in the 2.3 GHz band. That spectrum will be able to support immediate delivery of 4G services.
The UK's big four mobile network operators, EE, Vodafone, O2 and Three, will compete for the spectrum being auctioned off with a fifth company, Airspan Spectrum Holdings, although caps that Ofcom has imposed limit the amount of spectrum that EE and Vodafone will be able to acquire.
Ofcom previously said that the outcome of the spectrum auctions may not be known for "a number of weeks".
Philip Marnick, Ofcom’s spectrum group director said: "Our job is to release these airwaves quickly and efficiently, and we want to see them in use as soon as possible. We are glad the auction is now underway. This spectrum will help improve people’s experience of using mobile broadband today, and also help companies prepare for future 5G services."
Telecoms expert Daryl Cox of Pinsent Masons, the law firm behind Out-Law.com, said Ofcom has pushed ahead with the auction after unsuccessful legal challenges were brought by Three and EE against the spectrum rules.
"The mid-frequency bands under auction are important 'work horse' bands to provide increased capacity for 4G and 5G mobile services," Cox said. "There are also plans to clear and auction spectrum in the 3.6-3.8 GHz band next year, with 3.8-4.2 GHz also under consideration. Beyond the mid-frequency spectrum, Ofcom has indicated that it will clear low-frequency spectrum in the 700 MHz band for improved coverage and user experience, and also high-frequency 'pioneer', or millimetre-wave, spectrum of 26 GHz and above to support high capacity, very low latency applications."
"Taken together, these frequency bands are expected to enable the various benefits of 5G, which are not limited to high speed mobile internet. These benefits also include exciting new use cases for internet of things, autonomous vehicles, remote controlled drones and augmented reality," he said.
Earlier this month, Cox said that the way in which content is consumed and provided will be revolutionised by the introduction of 5G technology but that there are regulatory and commercial challenges still to be overcome before 5G services go mainstream.