Red Bull's Helmut Marko says a divide between Toro Rosso and James Key with regard to the junior bull squad's future technical plans enticed the British engineer to move to McLaren.
Key's transfer to the Woking-based outfit has been in the works for several months, with Toro Rosso - or rather Marko - refusing to grant the team's technical director an early release from his contract with the Faenza-based outfit.
However, following the conclusion of negotiations with McLaren, the Red Bull motorsport boss has confirmed Key's upcoming departure, and revealed the dissensions that led to it which were partly rooted in the energy drink company's desire to achieve more synergies between its two F1 teams.
"Dietrich Mateschitz gave us the task to form a closer relationship with Toro Rosso, and with that, one thing has to be avoided: having different people at both teams doing the same job," Marko explained to Motorsport-Total.com.
"We had a plan for this, a plan which hasn't been completed yet, but a plan James Key didn't agree with.
"In the end, I think it's easier to not keep him and I think this is the best solution."
With Red Bull Racing and Toro Rosso sharing more common technical ground, Key likely questioned his long-term position at Faenza, and subsequently opted for the McLaren alternative.
Marko also revealed that Key would not be replaced at Toro Rosso, given the synergies now in place between Milton Keynes and Faenza.
"We won't try to fill the vacancy for technical director at Toro Rosso with someone else," said the Austrian.
"The function is no longer necessary as Red Bull's concept will be used [at Toro Rosso]."