The group is called Harbour and has seven founding agencies including George & Dragon, Goodstuff, Oliver and Talon.
Also part of the mix are: Dac, the digital media, search and geolocation agency, Just So, the integrated creative and film production company, and Scramble, the sound and vision agency.
Harbour will provide these agencies with a centralised team for areas such as finance, HR, IT, legal and travel. The idea is that the different shops can also call on each other when pitching for new business.
Hammersley said there will be more agencies joining the fold over the next few months.
He is not taking a stake in the companies because "it’s too complicated". However, Hammersley explained that further along in the process Harbour may help start-ups and in return take a stake in the business.
Hammersley said: "We’ve got to a place where 75% of the global industry is controlled by six companies. There’s no reason to have that concentration of ownership.
"Some clients see reassurance in scale so the logic is to help independent agencies survive through connected offers.
"The industry needs this and it will help level the playing field."
Kenny Jacobs, chief marketing officer at Ryanair, said: "Harbour represents a great way to help level the playing field between the independent agency sector and the big networks. That’s good for clients as it widens and improves our agency options."