It's not uncommon these days to be asked to fork over your Social Security number when you visit a doctor, apply for a loan, set up a banking account or even open up a gym membership.

The use of Social Security numbers to identity consumers is rampant, and with it, consumer advocates say, comes greater risks of identity theft.

"SSNs generally are considered the most valuable piece of consumer data for identity thieves," according to the Federal Trade Commission, which wrapped a two-day workshop yesterday to gauge the private use of Social Security numbers and its role in identity theft.

Identity thieves use the numbers to open new accounts, obtain new driver's licenses, access existing accounts or use it for medical identity theft, the FTC said.

While some requests for SSNs are legitimate, Consumers Union notes that not all businesses who ask for the SSN have a clear need for the number. "Consumers may be asked for the SSN to join a health club, to donate blood, to purchase or activate a cell phone service, or simply to write a check," the non-profit advocacy group told the FTC.

Although the Social Security Administration advises consumers when asked for their social security number by a business to inquire why the number is needed; how it will be used; what happens if the consumer refuses to provide the number; and what law requires the consumer to provide the number, federal law neither requires businesses to provide this information nor prohibits solicitation of the number or denial of the transaction if the customer refuses to provide it.

Moreover, for most businesses outside of the financial sector, no federal law requires the protection of this sensitive information once a business has collected it nor restricts how businesses may use or reuse the SSN or to whom they provide it, including even commercial resale of the SSN. The widespread and often unnecessary solicitation, use and commercial sale and purchase of SSNs, coupled with inadequate safeguards make consumers particularly vulnerable to identity theft.

The Privacy Rights Clearinghouse offers these tips if a private business requests your SSN:

Leave the space for the SSN on the form blank or write "refused" or "N/A" in that space.

Speak to someone in management or write to the business and explain why you do not want your SSN used to identify you.

Insist that the company document its policy of why they are requiring a SSN. If a written policy cannot be found or too much time is taken looking for one, maybe the business will allow you to use an alternate number.

Ask why your SSN is requested and suggest alternatives such as using your driver's license number.