NEW YORK, Aug 24 (Reuters) - Billionaire investor William Ackman, who last year launched the biggest-ever special-purpose acquisition company, said on Tuesday he remains committed to finding a merger partner for his blank-check company Pershing Square Tontine Holdings Ltd (PSTH.N) (PSTH). "Our plan to return cash to shareholders once SPARC is approved does NOT in any way mean that we are walking away from PSTH and giving up on completing a deal," Ackman wrote in the Pershing Square Holdings interim financial statements. Ackman made the statements just a few days after he told investors he would return the $4 billion raised last year if regulators give him approval for a special-purpose acquisition rights company (SPARC). The news came days after Pershing Square Tontine Holdings was charged with failing to have registered as an investment company, a lawsuit Ackman called meritless. "We remain committed to finding a transaction for PSTH," Ackman wrote on Tuesday, adding, "If we have not done so by the time SPARC is approved, we will then continue to pursue a business combination, on behalf of SPARC rather than PSTH." Ackman created the SPARC as part of a deal to buy a stake in Universal Music Group (UMG), which he proposed earlier this summer for Pershing Square Tontine Holdings but abandoned amid regulatory questions. Instead his hedge funds promised to purchase between 5% and 10% of UMG by mid-September and have already purchased a 7.1% stake, the financial statement said. To help raise cash for the purchase, Ackman said he sold his stake in Agilent Technologies Inc (A.N), his portfolio"s second-best performer in the first half of the year. He said again he thinks regulators should grant approval to his SPARC "within a reasonable time frame, that is in months not years" and said investors could help the process by contacting the U.S. Securities and Exchange Commission and the New York Stock Exchange.