McLaren appear to be edging towards an engine deal with Renault although the team insist a decision on their 2018 power supply hasn't been made yet.
McLaren are believed to be intent on divorcing from Honda but their plans could be dependent on Toro Rosso agreeing a deal with the Japanese manufacturer that would enable Honda to stay in the sport even if they split from McLaren.
Renault bosses Jerome Stoll, Cyril Abiteboul and Alain Prost, accompanied by F1 boss Ross Brawn, held talks inside McLaren's motorhome after Friday practice for the Italian GP.
"It is very complicated," McLaren team boss Zak Brown said. "There are a variety of pieces to the puzzle - some in our control, some not.
"If everything was done we would have a decision and an announcement to make. There are some challenges ahead of us that we need to navigate."
Speaking to Sky Sports News, Brown added: "It can still go either way. It's probably one of the biggest decisions McLaren have ever had to make."
The American later added: "We don't have an agreement in place with anybody."
Although Toro Rosso have reputedly set a deadline of this weekend for a decision to be made on their own engine deal, Brown says his team have until mid-September to make their choice.
But the McLaren boss was tight-lipped on his ongoing discussions with Honda. "I am not going to comment on any meetings with Honda," he said.
Honda are thought to provide McLaren with funding worth at least £50m a year. But after three years at the back of the grid from where they have failed to land a single podium, Brown says the team's only priority for 2018 is being as competitive as possible.
"The best thing for McLaren is to make the best sporting decision possible," he said. "We are here to win so we are going to make a sporting decision.
"We can't afford not to be on the podium. It's a big decision that has lots of elements to it and economics are one. But fortunately we have extremely committed shareholders which means we can make a sporting decision and deal with the economics."
Were McLaren to split with Honda and join forces with Renault, it is expected that Fernando Alonso would stay with the team. But Brown denied that the Spaniard had warned McLaren he would leave unless they divorced Honda.
"Fernando has not given us any ultimatum, that it's 'them or us', at all. His decision will be based on whether he thinks we will be competitive or not. He's not been engine-specific in his desires.
"Negotiations are ongoing and they are going very well. But you have to have a Plan B and a Plan C in the event you don't reach an agreement, and we have those plans."
But were Alonso to leave, British youngster Lando Norris, who starred recently during testing in Hungary, will not take the two-time world champion's place.
"Our current plan is to put him in Formula 2," said Brown. "I certainly think he's fast enough to be in Formula 1 but we don't want to rush things.
"He's definitely our star of the future and we won't put him in another Formula 1 team at this point."
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