WASHINGTON—Capitol Hill’s biggest Wall Street critic is raising fresh concerns that large financial firms backing a new instant-messaging service may use it to circumvent regulatory scrutiny.

Sen. Elizabeth Warren (D., Mass) on Monday sent letters to six financial regulators saying she is troubled that the big banks and other financial firms backing Symphony Communications Services LLC may use the new tool to skirt regulatory and legal requirements, as well as escape enforcement action by the Consumer Financial Protection Bureau and other regulators.

She also pointed to a letter sent last month to Symphony by the New York Department of Financial Services, which asked the startup for details of its data retention and deletion capabilities, among other features. The New York regulator pointed to promotional materials from the company that touted its “Guaranteed Data Deletion” and encryption protections.

The concerns raised by Ms. Warren and the New York regulator center on the ability of regulators and law enforcement personnel to access the sorts of written communications that aided investigations into a series of alleged rate-fixing scandals that have embroiled major financial firms around the world. As Ms. Warren notes in her letters, in the case of the manipulation of the London interbank offered rate, or LIBOR, bankers “used chat rooms and text messages to coordinate their activities, and it was the trail of such messages that permitted regulators both to discover and prosecute these financial crimes.”

Symphony was launched last October with the backing of 14 of the world’s biggest banks and money managers, including Goldman Sachs Group Inc., Bank of America Corp., BlackRock Inc. and Citadel LLC. The firms wanted to find a way to enable employees to trade messages instantly and securely, as well as provide an alternative to the messaging system provided by Bloomberg LLP. The Journal reported last month that Symphony is seeking another round of funding that could value the firm at as much as $1 billion.