WASHINGTON — A senior House Democrat has called for a wide-ranging federal investigation into Blackwater Worldwide, alleging that the private security contractor violated tax and labor laws by classifying its guards as independent contractors rather than company employees.

A spokeswoman for Blackwater, Anne Tyrrell, called the charges “completely without merit.”

“Blackwater’s classification of its personnel is accurate, and Blackwater has always been forthcoming about this aspect of its business with its customer, the U.S. government,” she said in an e-mailed statement yesterday.

But the chairman of the House Committee on Oversight and Government Reform, Rep. Henry Waxman, says Blackwater’s claims on its business status “appear dubious.”

In letters sent yesterday, Mr. Waxman asked the Internal Revenue Service and the Labor Department to investigate whether Blackwater defrauded the government of tax revenue and violated labor laws. Mr. Waxman also asked the Small Business Administration to determine if Blackwater violated federal regulations by claiming it was eligible for small business preferences.

“The implications of Blackwater’s actions are significant,” Mr. Waxman, a Democrat of California, wrote in a memorandum to his colleagues on the panel. “Committee staff have estimated that Blackwater has avoided paying or withholding up to $50 million in federal taxes by treating its guards as independent contractors rather than employees.”

Also, Mr. Waxman wrote, Blackwater’s claim as a small business has earned it more than $144 million in contracts, despite being one of the largest private military contractors and receiving nearly $1.25 billion in federal business since 2000.

Ms. Tyrrell said Blackwater “looks forward to continuing its cooperation with all inquiries that may result from these letters” and that “the company regrets the chairman’s decision to publicly air misleading information.”

Unlike other security companies operating in Iraq, Blackwater says the guards it trains, equips, and deploys to Iraq and elsewhere are independent contractors hired directly by the federal government and not company employees. Under American law, companies must pay Social Security and other federal taxes on employees.

Mr. Waxman raised the issue last year, after he obtained a March letter from the IRS that warned that the company’s classification of a security guard as an independent contractor was “without merit.” The IRS’s finding had been the result of an inquiry filed by a Blackwater guard.