Aug 2 (Reuters) - The U.S. Securities and Exchange Commission has opened a formal investigation into Sprint Nextel Corp's sales tax collection following a $300 million New York court case against Sprint, the No. 3 U.S. mobile provider.

Sprint said on Thursday in its quarterly filing with the SEC that the regulator had issued the order for an investigation on July 23, three days before the company' quarterly earnings conference call.

Sprint said it is cooperating with the SEC and does not expect these matters to hurt its financial position or the results of its operations.

The company, which said that the New York case is without merit, said in the filing that it could not predict the outcome or the timeframe for the conclusion of the SEC investigation.

New York State said in April that it was suing Sprint for more than $300 million and accused it of committing tax fraud by deliberately not collecting or paying millions of dollars of taxes for its cellphone services.

Sprint said in a separate statement that it had recently received a subpoena from the SEC in connection with its probe into its collection of a remittance of state and local taxes, including New York.

The company said that it believes its practices around state and local taxes on mobile wireless services are "in compliance with applicable law."

On June 14, Sprint filed to have the New York Attorney General's case dismissed. It said on Thursday that it "intends to continue defending the matter vigorously."

Sprint shares closed down 3 cents or less than 1 percent at $4.29 on the New York Stock Exchange on Thursday.

(Reporting By Sinead Carew and Jonathan Stempel)

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