Honda is to introduce the third upgrade of its power unit at Spa-Francorchamps this weekend for the Belgian Grand Prix.

However, as previously reported the Japanese manufacturer and its partner teams face a dilemma in terms of when exactly to introduce the new unit.

The races in Belgium and next weekend in Italy are both on power circuits where the latest upgrade could well be advantageous as Red Bull seeks to leapfrog Ferrari in the standings. However, these are followed by Singapore, where the Austrian outfit's chassis will allow it to take on Mercedes.

With an eye on the resultant penalties that will be incurred therefore, Honda has revealed that the new unit will only be available to Alexander Albon and Daniil Kvyat this weekend.

"We are introducing the Spec 4 version of our PU at this event," said Honda's technical director Toyoharu Tanabe. "As usual, we have focused on improving both reliability and performance, in the hope of achieving even better results with both teams in this part of the season.

"Our plan is that only Albon and Kvyat will run it at Spa, for strategic reasons, looking at the rest of the season as a whole," he added.

Having previously admitted that the timing of the introduction of the new unit was "complicated", Tanabe revealed that "we have two types of plan, long-term and short-term.

While Singapore is undoubtedly Red Bull's best chance of claiming a third win this season, should Max Verstappen not use the unit in Belgium or Italy, this would leave Russia and Japan as the next opportunities, and while Sochi is not the best place to start with a grid penalty, Honda would not want to compromise its home race at Suzuka.

The second upgrade was introduced in France, and has therefore contested five races thus far. While Honda is confident that unit is reliable enough for a couple more races, mindful of Singapore, and then Japan, in reality Sochi might well be the best option.

"After the race we review the situation, the condition of the PU," Tanabe previously told Motorsport.com, "maybe two or three races is the short-term. And the long-term means until the end of the season. It is very complicated."

"The driver swap between Albon and Gasly has no effect on how we operate at the race, and we continue to do our best to support all four of our drivers," said Tanabe ahead of this weekend's race. "As per the Sporting Regulations, Albon now takes on what was Gasly's PU usage in terms of all the components that make up the PU and vice versa."

Check out our Thursday gallery from Spa, here.