A frustrated Fernando Alonso has launched a damning assessment of McLaren partner and engine supplier Honda by declaring it is 'not ready to win' with a power unit that is losing upwards of 30km/h in a straight line.

Beset with engine-related issues since pre-season testing began in Spain last week, though the MCL32 completed 46 trouble-free laps with Alonso behind the wheel today, it served to demonstrate its evident lack of power and top speed relative to rivals.

Arguably an even bigger embarrassment to Honda than the poor reliability following the move to re-design almost the entire power unit ahead of the 2017 season, while Alonso is relatively happy with his McLaren chassis itself, he says the lack of top speed is hurting.

"The chassis everything feels good, everything feels under control. The car is responding well to changes and everything is working fine. I'm happy with the balance, I'm happy with how I attack the corner. I'm enjoying driving this car, so I don't think that we are too far back in terms of chassis side.

"We have only one problem: that is the power unit. There is no reliability and there is no power. I think we are 30 km/h down on every straight. When you are 30 km/h down on every straight, it is difficult also to have a feeling on the car. Everything feels good, but when you arrive to normal speed you don't know what is going to happen."

Revealing he expects an 'immediate reaction' from the team, Alonso goes on to clarify that he is referring to Honda solely, rather than McLaren.

"Well it's more a question for Honda. I have a lot of time. As I said, I am enjoying [it], I am preparing myself better than ever.

"I feel really strong driving this year with these cars, I can do also my driving style, my quick input on the steering wheel on entry in the old days, so I'm really enjoying. I'm feeling very strong, I'm feeling the strongest here, but I don't have the power. I have a lot of time."

With one day remaining in the car, Alonso has called on the team to pull together to ensure it can end testing without troubles to get a better idea of whether it can unlock more performance.

"Let's get together with Honda, push ahead together, because I believe we have potential to do a lot better than what we're doing now. We can't be 30/40 kph slower down the straights.

"If they have any problem, if there are differences between what happens in the test benches and on the track, whatever is happening, we have to overcome that together and complete as many laps as possible on the last two days of testing.