Shuttleworth started work at his as-yet-unnamed practice on Monday, after Arup boss Bob Emerson invited him to take some of the multidisciplinary consultant's space in its new Shepherd Robson-designed office in London's West End.
Foster and Partners project directors Sean Affleck and Jason Parker, as well as architect James Thomas, have decided to follow Shuttleworth and will join him at his temporary offices next month.
The moves are the latest blows to Foster's senior staff, as several project directors have also followed former director Robin Partington to Hamilton Associates in the past 18 months. Shuttleworth had also discussed the possibility of teaming up with Partington again, but decided to go it alone.
Shuttleworth said he had not decided whether the new practice would be named after him, as is the tradition with architects. He expected to start work on projects in two to three months' time.
He said: "I'm just talking to people and going to meet them at the moment. It is only week one."
Shuttleworth added that he had received 150 letters of support following his decision to quit Foster and Partners after 29 years.
One possible future client is luxury flat developer Manhattan Loft Corporation. Chief executive Harry Handelsman met Shuttleworth for lunch earlier in the week and said he was keen to work with him.
Industry sources had long speculated there was a rift between Shuttleworth and Foster. The two were reported to have fallen out over Shuttleworth's Building interview last January, in which he discussed his role in the design of some of the practice's most prestigious buildings.
Sunny Varkey's company, Global Educational Management Systems, is believed to have asked Hamilton to work up some generic designs for the schools, while it is thought that Mace is looking at four or five specific projects. Mace is not thought to have signed a framework contract.
GEMS is set to advertise for staff for the schools in the next few months, to teach children from aged three to late teens. Hamilton and Mace declined to comment.