The acting head of enforcement at the US Consumer Financial Protection Bureau has resigned, condemning what she described as an unprecedented assault on the agency's core mission under the Trump administration.

Cara Petersen, who served as acting enforcement director and had worked at the CFPB since its creation nearly 15 years ago, announced her departure in a scathing email sent to colleagues on Tuesday.

"I have served under every director and acting director in the bureau's history and never before have I seen the ability to perform our core mission so under attack," Petersen wrote in the email obtained by news agencies.

The resignation marks the latest high-profile departure from the consumer watchdog agency, which has been fighting for survival since President Trump installed Russell Vought, the White House budget office director, as acting leader in February. Petersen's predecessor, Eric Halperin, also resigned in February with his own critical email.

Under Vought's leadership, the CFPB has abandoned most of its enforcement cases, including major lawsuits against large banks over fraud on payment apps and deceptive tactics that prevented customers from receiving higher interest rates on savings accounts. The agency has also terminated settlement deals, allowing companies to retain money they had agreed to pay in penalties and customer refunds.

Last month, the bureau terminated an order requiring Toyota to refund $48 million to customers whom the carmaker had prevented from cancelling unwanted insurance products. Major enforcement cases against Capital One and Walmart have also been dropped.

"It has been devastating to see the bureau's enforcement function being dismantled through thoughtless reductions in staff, inexplicable dismissals of cases, and terminations of negotiated settlements that let wrongdoers off the hook," Petersen said in her farewell message.

The Trump administration has sought to cut the CFPB's workforce by approximately 90 per cent, though court orders have temporarily halted mass firings. Much of the agency's remaining staff has been placed on administrative leave whilst legal challenges continue.

Acting director Vought and chief legal officer Mark Paoletta have put most CFPB functions on hold since February, with enforcement attorneys recently asked to prepare "death memos" outlining why existing investigations should continue.

"It is clear that the bureau's current leadership has no intention to enforce the law in any meaningful way," Petersen wrote. "While I wish you all the best, I worry for American consumers."

The CFPB, created following the 2007-2009 financial crisis, has returned more than $12 billion to consumers harmed by banks, payday lenders and other financial companies since opening in 2011. Republicans have long criticised the agency as too powerful and lacking oversight, whilst Democrats argue it plays a vital role protecting consumers in financial markets.

A federal appeals court in Washington is yet to rule on the Trump administration's effort to overturn a court injunction blocking the mass firing of agency staff.

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