(Adds Theranos comment)

May 26 (Reuters) - Blood-testing company Theranos Inc was sued on Thursday, accused of endangering customer health through "massive failures" that misrepresented the accuracy and quality of its blood tests, according to court papers.

The proposed class action was brought in the U.S. District Court for the Northern District of California by the law firm Hagens Berman Sobol Shapiro LLP.

The complaint accuses the company of breach of contract, false advertising and consumer fraud, among other claims. It estimated that thousands of consumers could be eligible to join the lawsuit, citing Theranos' claims that it had performed more than 6 million tests.

In an emailed statement, Theranos called the lawsuit "without merit," and said it "will vigorously defend itself against these claims."

Theranos last week notified U.S. federal health regulators that it voided results from its Edison blood-testing devices for two years, the Wall Street Journal reported, citing a person familiar with the matter.

The company informed the Centers for Medicare and Medicaid Services that it has issued tens of thousands of corrected blood-test reports to doctors and patients, nullifying some results and revising others, the Journal reported.

Theranos once touted its Edison device as a ground-breaking technology able to test blood from just a pinprick.

The complaint focuses on materials published by both Theranos and Walgreens Boots Alliance, which had agreed to offer blood testing from Theranos' Edison device at dozens of wellness centers in Walgreen's stores in Arizona and California.

CMS said in January that the blood-testing company's practices violated several clinical-laboratory regulations, jeopardizing patient health and safety.

Theranos, which recently expanded its board to include Dr. Fabrizio Bonanni, a former Amgen Inc and Baxter International Inc executive, has been under investigation by several U.S. regulators in the recent past.

(Reporting By Deena Beasley; Editing by David Gregorio, Bernard Orr)