SPRINGFIELD — Illinois residents have been avoiding the taxman for years by shopping online, a strategy that will become harder under a new law that requires online merchants to collect sales tax on purchases made through business partners based in the state.

Supporters say the law signed Thursday by Gov. Pat Quinn will help level the playing field between brick-and-mortar stores and online businesses. Critics argue it will drive companies out of Illinois.

Many online retailers, such as Amazon.com and Overstock.com, do not charge a state sales tax.

Illinois residents who buy from those retailers are supposed to estimate their tax debt each year and pay it to the state voluntarily. Few people do.

Under the new state law, sales taxes will be charged on purchases made through Illinois “affiliates” of those online companies.

The law is good news for communities that have seen the sales tax income they rely on eroded by online commerce.

“In terms of the spirit of the law, there are many, many municipalities that rely on sales tax revenue for their general funds … this could bring relief,” said Normal Mayor Chris Koos.