Mercedes technical director James Allison has cast doubt on the impact the new 2021 Formula 1 cars will have on improving racing.

The sport's bosses have been working in conjunction with the FIA and teams to develop a design that allows drivers to follow closer and promote more overtaking by simplifying aero and increasing ground effect.

Mercedes has been one of the more vocal critics of the overhaul during the process, suggesting rules stability is what's needed to bring teams closer together.

And Allison says the bold predictions of the new cars only losing five per cent downforce compared to 50 per cent now, might be too good to be true.

“They have very worthy objectives behind them, but very difficult objectives to actually realise,” he told Crash.net.

“The proof of that pudding will be in the eating. We’ll find out in 2021 how successful the overall package is in delivering on those very worthy but very difficult goals."

Backing up his warning, the Briton, formerly of Renault and Ferrari, points to how F1's designers won't be interested in those goals when creating their designs.

“That doesn’t really, at this point, interest a team any longer," he admitted.

"A team just has the regulations in their hands and you’d be a weird F1 engineer if you weren’t excited by a fistful of new regulations to digest, to look for an opportunity within and go off to try and make the fastest car you can from them."

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“For us, probably the work of the last years - and particularly the climax of the last few weeks and months – has been a bit painful," he conceded.

“But we are at the point now where it is the fun bit. You’ve got the pamphlet in your hand and you set to work with it.”