Everybody wants it: your bank, health club, utility company, sometimes even the guy at the video store.

That nine-digit number that started as a way for the Social Security Administration to track workers’ earnings and benefits now is routinely requested by all sorts of businesses and groups.

“This is not supposed to be a national identification number … but that’s what it has become,” said Cheryl Hystad, executive director of the Maryland Consumer Rights Coalition.

A Social Security number legally must be divulged in limited situations, but for many years consumers readily rattled off their numbers to anyone who asked. With the rise of identity theft, it has become clear a Social Security number is the key to your finances. If it falls into the wrong hands, the culprit can assume your identity and establish accounts in your name.

Consumers can protect themselves by controlling when they give out their number. That can mean refusing to disclose a number when there’s no legal reason to do so, or even walking away from doing business with a company that won’t take no for an answer.

“You have a lot more power than you realize,” said Scott Ksander, assistant technology professor at Purdue University.

Ksander, for instance, said his utility company demanded his Social Security number when he recently reported that his gas meter was damaged by a storm. Ksander resisted, and the utility relented.

The first Social Security numbers rolled out in late 1936. In 1961, Congress permitted the Internal Revenue Service to use the numbers to identify taxpayers.

Besides those two agencies, you must also provide it to those required to collect income and tax-related information on behalf of the IRS. That includes banks and financial institutions that give interest income information to Uncle Sam, and employers that report earnings and Social Security taxes.

As part of a federal law in the mid-1990s to improve collection of child support, states require Social Security numbers on license applications, whether it’s for hunting or fishing, marriage, driving or professional licenses.

As a result of the Patriot Act of 2001, banks must verify the identities of new customers, and most do so by using Social Security numbers, said John Hall, a spokesman with the American Bankers Association.

Congress in 1974 limited the use of the numbers by federal agencies because of privacy concerns, but the law didn’t apply to private businesses or states, experts said. The use of Social Security numbers became widespread, appearing on drivers’ licenses, insurance cards, student IDs and even gym memberships.

Today, it’s not just businesses that want the number, but schemers developing all sorts of ways to wangle the information out of consumers. Most recently, federal courts warned about bogus phone calls accusing people of shirking jury duty while trying to extract their Social Security numbers.

Once a number is obtained by those intent on fraud, the damage can be swift.

Angela Butler of Dayton, Md., said her teenage daughter’s identity was stolen two years ago by thieves who had pilfered her purse containing her driver’s license and paycheck with her Social Security number on it.

Butler said she became aware of the theft about two weeks later when an $877 bill arrived for cell phones and monthly calling plans. After getting her daughter’s credit report and digging further, Butler discovered that two $25,000 Chevy trucks had been purchased in the 18-year-old’s name. It took nearly a year to clear up the problem, Butler said.