McLaren chief Ron Dennis believes customer teams will never be able to win a Formula 1 championship under the current engine regulations.
Ahead of his team's switch to Honda power units next year, Dennis said earlier this month that works teams have such a huge advantage in getting the most out of the new hybrid systems.
Reflecting on his team's campaign as a Mercedes customer, McLaren's Group CEO said: "The one thing that jumps at you, if you look at all the qualifications this year, is the time difference between the Mercedes-Benz works team and other teams.
"And by and large it is always in excess of one second, putting aside the pace that they can generate in a grand prix when they are on their back foot.
"My opinion, and it is an opinion held by many people within our organisation, is that you have no chance of winning the world championship if you are not receiving the best engines from whoever is manufacturing your engines.
"And a modern grand prix engine at this moment in time is not about sheer power, it is about how you harvest the energy, it is about how you store the energy."
He added: "Effectively, if you don't have the control of that process, meaning access to source code, then you are not going to be able to stabilise your car in the entry to corners etc., and you lose lots of lap time.
"Even though you have the same brand of engine that does not mean you have the ability to optimise the engine."
Dennis said the conviction that having a works engine was necessary was behind his team's switch to Honda - as he made clear that his outfit will be its number one priority even if other customers come on board.
"You have to start by putting yourself in a position where you have the best engine available.
"That is what we have done over I would say over the approaching years - read into that what you want. It is not months.
"We have had a great partnership with Mercedes but we intend to hit the ground running with Honda and that is the first step."