The FIA has confirmed that Formula 1 manufacturers will not be able to offer second-tier engines to its customer teams next season, unless all teams now agree to change the rules.
There had been suggestions that Ferrari could offer 2015-specification engines, either to Red Bull or Toro Rosso, because it is now too late to ramp up production of the power unit that will be used by the works teams next year.
But in regulations published by the FIA on Wednesday, it is made clear that as things stand Ferrari will not be able to offer anything but identical engines to its customer teams.
Article 23.5 of F1's 2016 Sporting Regulations said: "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."
Although this rule prevents different specifications of engines, it does not cut off the possibility of manufacturers using better software mapping or fuel products to ensure a competitive advantage over their customers.
While that rule would appear to shut off any options of Red Bull being given second-tier engines, it is still possible that the regulations can be changed if all teams agree.
Article 1.2 of the rules says: "These Sporting Regulations were published on 30 September 2015 and may only be changed after this date 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."
With F1's engine manufacturers meeting the FIA for a rules summit on Thursday, it is possible that there could yet be an agreement that would allow some compromises, if it helped either of the Red Bull teams or Ferrari.
F1 teams have already expressed some support for a rule tweak to allow 2015 engines to be used next year, but this still needs discussion at the next F1 Commission meeting.