Chadwick, 21, won this year’s inaugural championship and $500,000 prize money and is returning next year when points are being awarded for the first time towards a Formula One super license.
Another title would bar her from competing in 2021, however.
“W Series has been granted FIA super license points eligibility, and one of the criteria attached to that is that the winning driver … may not compete in consecutive W Series championships,” said racing director Dave Ryan.
“That restriction will apply going forward,” he added in an interview conducted by the series.
“It does mean that whoever is W Series champion in 2020 will not be permitted to take part in the 2021 W Series championship, and so on.”
The ruling did not apply to this year because license points were not available.
The W Series has announced the names of 18 of the 20 drivers who will compete next year, and Ryan said the final two depended on scheduling issues that the drivers wanted more time to resolve.
The series uses identical 1.8-liter Formula Three cars and aims to help female racers climb the male-dominated motorsport ladder towards Formula One, which last saw a woman start a race in 1976.
Season two looks set to feature more than the current six races, but Ryan said the calendar had yet to be finalized.
“We’ll still race alongside our great partners DTM (the German Touring Car championship), for the majority or perhaps even the totality of our race fixtures,” he said.
There has been talk of W Series expanding outside of Europe to serve as a support race at next year’s U.S. and Mexican Formula One Grands Prix.