Renault has warned Red Bull that it will not offer an extension to the FIA's May 15 deadline for a 2019 engine deal, giving the former world champions less than two weeks to make a decision on its power unit supply for next year.

Red Bull faces a straight choice between renewing its contract with Renault or teaming up with Honda next year, and held its first formal talks with the Japanese manufacturer at the Azerbaijan Grand Prix last weekend.

Current estimations put Renault's engine ahead of Honda's in terms of power output, but both remain some way short of class leaders Ferrari and Mercedes. Planned upgrades for both manufacturers at June's Canadian Grand Prix are likely to give the clearest indication of how competitive each engine will be next year, but as things stand Red Bull will not have the luxury of analysing that data before it makes a decision.

Under the regulations, engine manufacturers have to confirm their customers for the following year by May 15, although that deadline can be extended if the governing body and all engine suppliers are in agreement. That loophole facilitated McLaren's late decision to switch from Honda to Renault last year and Toro Rosso from Renault to Honda, but Renault Sport managing director Cyril Abiteboul said the May 15 deadline must be adhered to this year.

"We have to [stick to it]," he told ESPN. "We have to for a very simple reason, which is that we need to order parts.

"The problem is that people tend to amalgamate what happened last year when we changed very late our customers, but we only swapped customers. So actually we decided to allocate, to deviate, engines that were produced for Toro Rosso and to make them available for McLaren, but the parts for those engines were already ordered. I have to order now, the longest lead time items for next year, so what do I do: three or two?

Renault and Red Bull have had a difficult relationship in recent years. Dan Istitene/Getty Images

"Frankly, I discussed that question with Red Bull on a regular basis. It's not that we want to be awkward -- there's no tactics -- but we simply must define the quantity. That's why it's in the sporting regulations, and what we discussed with the FIA regarding obligation of supply from suppliers to the customer teams, we agreed on the deadline of May simply because that's the life of the project.

"Red Bull will have to cope with it."

Abiteboul said Renault would happily continue with just McLaren as an engine customer next year.

"It's important to have one customer -- I would not want to lose two customers, really not -- but that's also one of the reasons why we've done what we have done with McLaren, which is to have a long term contract with them until 2020.

"We are already in discussions with McLaren with what could happen beyond that point because there is a general feelings that we can achieve great things together. But, frankly, it's not a problem from moving from two customers to one customer."

What went wrong with the Red Bull/Renault partnership?

After winning four consecutive drivers' and constructors' championships together between 2010 and 2013, the Red Bull/Renault partnership became strained when a new set of engine regulations were introduced in 2014 and Renault's turbo-hybrid underdelivered.

Abiteboul believes the biggest single issue was sticking to a strict customer/supplier relationship rather than committing to a works partnership with Red Bull to rival the factory teams of Mercedes and Ferrari. He also thinks Renault's engineers suffered by not having a direct link to the race track and missed out on developing power modes to rival the top two manufacturers as a result.

Cyril Abiteboul believes Renault's recent relationship with Red Bull has been full of missed opportunities. Mark Thompson/Getty Images

"What we did not manage to do is understand and appreciate the complexity of the 2014 regulations and appreciate together that the model of a customer on one side and a supplier on the other side was completely blown away by the new regulations. That's why Mercedes and also Ferrari were the two teams to take advantage of that -- they are great teams, have great facilities, well-resourced good people but it's also because they are fully integrated.

"That level of integration, we didn't see that coming -- neither Red Bull nor Renault. That was our common mistake, that we didn't manage to support and identify and to react and correct. So we have been a little bit complacent with the model in which we wanted to collectively operate. That was not appropriate, that was not suitable or adapted to those new regulations.

"I think by keeping us as a supplier and by us keeping them as a customer at arm's length, it has also undermined the capacity for Viry -- our engine base -- to be in close contact with the racing. All the discussion that we have about oil combustion and qualifying modes are things that we have never thought of and that is a direct consequence of the way we are working with Red Bull, and a direct consequence also of the fact that we had lost contact by not being a works team, or not being associated with a team that was considering us a complete partner.

"We've lost ground by not being in the racing environment. That's one of the things that we are re-building now that we have a works team."