The FIA has granted Ferrari permission to supply a 2015-spec Formula 1 power unit to Toro Rosso next season.
Following Renault's troubles with 1.6-litre V6 turbo-charged hybrid systems, both Red Bull and Toro Rosso served notice on the French manufacturer it no longer wanted a supply for 2016.
While Red Bull is now set to stick with Renault, Toro Rosso opted to reunite with former supplier Ferrari after previously working with the Scuderia from 2007-13.
For the deal to go through, Toro Rosso has agreed to using this year's Ferrari engine in next year's car.
As the current regulations do not allow for two different specifications of power unit from one manufacturer to be used in a single season, special dispensation has had to be applied for to the FIA.
Following a meeting of the World Motor Sport Council in Paris, this has now been formally ratified.
A FIA statement read: "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."
MANUFACTURERS GET MORE TOKENS
The FIA has also opened up in-season development of power units from 2016 and beyond, as previously revealed by Autosport, with a decision taken to increase the number of tokens available.
For the introduction of 1.6-litre V6 turbo-charged hybrid systems last year, the FIA opted for a sharply declining development curve.
In 2015, 32 tokens were available, reducing to 25 next year, then 20, 15 and three respectively for '17-19.
But now, given the struggles of the likes of Renault and Honda in their attempts to play catch up with Mercedes, the FIA has altered the figures.
For next season there will again be 32, followed by 25, 20 and 15 across 2017-19.
It means the areas that were due to be closed off for development in 2016 will remain open, namely the upper/lower crankcase, valve drive, crankshaft, air-valve system and ancillaries drive.
The WMSC also determined any new engine manufacturer entering F1 will be allocated just 15 development tokens in its first year, with 32 in the second.
There has also been one further alteration to the technical regulations, with the padded area around a driver's head to be increased for improved safety.