After motor racing's governing body accepted the current engine rules had not specifically stated when engines had to be homologated this year, it allowed car manufacturers to use engine development tokens throughout this season.

But with the FIA well aware that it did not want an open house on engine development, Motorsport.com has learned that the rules have been changed for 2016 to ensure that engines are fully homologated by February 28.

No changes to the power units will be allowed after that date, and any tokens unused over the winter will be lost.

Opening the door on development

That situation has prompted unease from Mercedes' rivals, who believe that forcing them to rush through all their engine upgrades before the start of the campaign will not only cost more but put them at risk of not being able to catch up over the course of the season.

It is understood that discussions have now begun between the engine makers about agreeing on a change to the rules to allow limited in-season tweaks for 2016.

F1's governance structure means that for such a rule change to happen it will require unanimous approval, and currently Mercedes is not in favour of opening up the rules.

Renault F1 chief Cyril Abiteboul is hoping that his company's rival can be won over though.

"Given the amount of work that we have to do, we will like to be able to use our tokens in a free manner," Abiteboul told Motorsport.com

Cost fears

Although fears of rising costs have been cited when it comes to stopping in-season development, Abiteboul actually thinks the money issue is a red herring.

"I don't see any connection between token in-season development and budget, because you still have the token system [outside of the season]," he said.

"We are not saying you should be able to do whatever you want. We are saying you can introduce when you want within the limits of the token allocation."

He added: "There is something to be discussed in terms of the weight of the tokens and the weight of the changes, I am open to that.

"But I am not open to the stop and go that the freeze during the season and open window during the winter is causing for our facilities.

"It is really not good for the general budget and resource management within our different factories."

No surprise with Mercedes stance

Last season, Ferrari had no option but to challenge the FIA over the wording of the engine homologation regulations because Mercedes would not agree to relaxing the freeze.

Although not happy Mercedes is resisting change again, Abiteboul fully understands why there may be competitive reasons behind its stance.

"Mercedes has every reason to try to slow us and prevent us from catching up," he said. "That is fair and it is part of the competition.

"But it is also part of the competition to try to do something different. Mercedes thinks that it is more expensive [to allow in-season development], but philosophically I genuinely doubt if this is the case if the token system is well managed."