Toro Rosso's plan to use 2015 Ferrari engines next year has been approved by the FIA.
The Faenza-based outfit has had in place for several weeks an outline agreement with the Maranello team to use year-old engines in 2016, but it could not be signed off until its Red Bull bosses were clear they wanted to stay in F1.
With Dietrich Mateschitz now wanting to keep his F1 teams running, and Red Bull close to announcing a plan to run rebranded Renault engines, Toro Rosso is clear to sort out its power unit plans.
The move to run 2015 Ferrari power units has needed a rule change, however, with manufacturers previously unable to run two types of engines in F1.
But at the FIA World Motor Sport Council meeting in Paris on Wednesday, the FIA said running year-old engines would be allowed, and Ferrari could now supply a fourth team.
Toro Rosso will join Ferrari, Sauber and Haas as the Maranello outfits fourth team.
The FIA said: "Power Units homologated in previous seasons may now be re-homologated. Previously no manufacturer could supply more than one specification of PU.
"The World Motor Sport Council was also advised that the FIA had agreed for Ferrari to supply a fourth customer team with a 2015-specification Power Unit in 2016."
Toro Rosso has not yet made any official announcement about its engine plans, but could do so imminently once Red Bull's own situation is made clear.