CHICAGO, Feb 7 (Reuters) - A U.S. court has nullified the government's latest approvals of certain agricultural weedkillers sold by Bayer, BASF and Syngenta, fueling uncertainty among farmers who spray the products on soybeans and cotton genetically engineered to resist them.

Environmental activists cheered the court for halting use of the dicamba-based herbicides, which are known to drift away and damage crops that cannot tolerate the chemical.

Some farm groups and agribusinesses said the ruling, if enforced by the federal government, risks hurting farmers financially and reducing options for fighting weeds that are increasingly developing resistance to a limited number of herbicides.

U.S. District Court Judge David Bury in Arizona this week vacated the Environmental Protection Agency's registrations of dicamba-based weedkillers from 2020, saying the agency violated procedures mandating public input. The ruling affects Bayer's XtendiMax, BASF's Enginia and Sygnenta's Tavium, commonly used herbicides on U.S. farms.

Bayer soybeans that resist dicamba-based herbicide are the No. 2-most planted soybeans in the United States.

The companies said they disagreed with the ruling and were waiting for direction from the EPA. The EPA said it was reviewing the decision.

"Most soybean and cotton farmers have made seed and chemistry purchase decisions and, in some cases, are preparing to plant their 2024 crop in the coming weeks," BASF said. "This order may threaten the livelihoods of soybean and cotton farmers who rely on over-the-top dicamba to control resistant weeds."

The ruling bars farmers from spraying the dicamba products in the upcoming growing season unless the EPA allows already-shipped batches to be used, said Meredith Stevenson, staff attorney for the Center for Food Safety. The center called the decision "a vital victory for farmers and the environment."

In June 2020, a U.S. appeals court blocked dicamba-based herbicide sales and ruled the EPA understated risks related to its use.

The EPA, under former President Donald Trump, subsequently said farmers could use their existing supplies before it eventually reauthorized use again with new restrictions in October 2020.

Under President Joe Biden, the agency in December 2021 questioned whether dicamba weedkillers could be sprayed safely on soybeans and cotton without posing "unreasonable risks" to other crops. (Reporting by Tom Polansek in Chicago Additional reporting by Clark Mindock in New York Editing by Matthew Lewis)