The DTM will lose a quarter of its grid next year, with all three manufacturers dropping from eight to six cars.
Following discussions during the Hockenheim season finale, official material from Audi confirmed "it is now official that only 18 instead of 24 cars will race in 2017".
Mercedes has also acknowledged the drop in a statement, adding its six cars will be run in-house under the HWA umbrella, with ART Grand Prix and Mucke Motorsport exiting the series, while BMW is yet to comment.
Autosport understands that the move has been motivated by a desire to make DTM more affordable for its existing manufacturers and a more attractive proposition for any prospective entrants.
Audi's 2013 DTM champion Mike Rockenfeller said that he did not believe the reduction would have a negative impact on the series.
"Of course, every driver and every fan wants as many cars as possible on the grid, but I think the focus in the races is anyway on the top third," said Rockenfeller.
"It's crucial that the races are exciting with many overtaking manoeuvres and the cars are fascinating.
"And in these aspects the DTM is on the right path."
BMW has already potentially finalised its trimmed driver line-up, with Martin Tomczyk and Antonio Felix da Costa leaving the DTM to represent the brand in GT racing and Formula E respectively.
Audi's Timo Scheider is retiring, while former stablemate Edoardo Mortara has confirmed his move to Mercedes.
All three brands tested at Oschersleben last week, starting work to develop spec components to be introduced in 2017, as part of the DTM's new Class One technical platform, with revised aerodynamics to follow at a later date.