India captain Virat Kohli says he is strongly against plans to reduce Test matches to four-day affairs.
The ICC is considering the idea as they ponder the international calendar for 2023 and beyond.
Four-day Tests would free up space in the schedule and also provide more rest for players, but it is a plan that has split opinion within the game.
The ECB say it is are in favour of the proposals, while captain Joe Root also gave his backing in an interview with the Sky Sports Cricket podcast.
However, while Cricket Australia say it will consider four-day Tests, captain Tim Paine, as well as team-mates Travis Head and Nathan Lyon, have criticised the plans.
The BCCI are yet to reveal its stance on the issue, but Kohli was forthright in his view when he said: "I am not a fan.
"I think the intent will not be right because then you will speak of three-day Tests. Where do you end? Then you will speak of Test cricket disappearing. I don't endorse that at all.
"I don't think that's fair to the purest format of the game. How cricket started initially and five-day Test matches was the highest you can have at the international level. According to me, it shouldn't be altered."
Optional four-day Tests were approved by the ICC in 2017, with England playing one against Ireland in July last year.
Day-night Tests have also been given the green light in recent years, but Kohli says that is where alterations to the game should end.
"I was asked about the Hundred - I said I'm not going to try myself in another format because there's already so much going on," Kohli said.
"Day-night cricket is the most that needs to be changed about Test cricket, according to me. Then you're purely going to be talking about getting the numbers in and entertainment.
"Day-night is another step towards commercialising Test cricket and creating excitement around it but it can't be tinkered with too much."