Formula 1's governing body, the FIA, has ruled out the possibility of customer squads using year-old power units from 2016.
Manor Marussia, which has signed a current-year deal with Mercedes for 2016, currently uses 2014 power units from Ferrari, having secured a last-minute return to the grid at the start of the year.
Both Red Bull and Toro Rosso are in search of power units for next year, with the Italian outfit having edged closer to a year-old deal with Ferrari.
However, the FIA updated the Sporting Regulations this week to rule out such a possibility - though the rule does not cover different software settings or fuel provider.
"Only power units which are identical to the power unit that has been homologated by the FIA in accordance with Appendix 4 of these regulations may be used at an Event during the 2016- 2020 Championship seasons," read Article 23.5 of the 2016 Sporting Regulations.
The Sporting Regulations can still be changed, though only "with the unanimous agreement of all competitors entered in the 2016 Championship, save for changes made by the FIA for safety reasons which may come into effect without notice or delay"