The US Secret Service has "deep flaws" that need to be resolved urgently or more assassination attempts like the one at Donald Trump's rally will happen, a damning report says.

An independent panel tasked with investigating the 13 July shooting in Butler, Pennsylvania, released its findings on Thursday, and said the organisation had become "bureaucratic, complacent and static".

It called for an overhaul of its leadership in the 52-page report, and said a "number of specific failures and breakdowns" enabled the attack against the Republican presidential candidate.

The Secret Service has already acknowledged failures on its part, and its director resigned weeks after the shooting.

In a statement on Thursday, its acting director Ronald Rowe said the agency would carefully examine the new report.

"We have already significantly improved our readiness, operational and organisational communications and implemented enhanced protective operations for the former president," he said.

But in an internal memo to agency staff, Acting Director Rowe said he had "reservations" about the recommendations, according to CBS News, the BBC's US partner.

He wrote: "I am deeply concerned about the unintended impact on agency morale."

In the independent report, which was drafted by state and national law enforcement officials, the panel praised the agents who risk their lives to protect many of the country's highest-ranking officials but noted several leadership and cultural failures.

These included a "troubling lack of critical thinking" among staff and a reluctance to "speak up".

The agency's issues, the report said, were "systemic or cultural" and it called for "fundamental reform", including removing some of its top leadership "as soon as possible".

"Without that reform... another Butler can and will happen again," the panel wrote to Department of Homeland Security Secretary Alejandro Mayorkas, who oversees the organisation.

President Joe Biden ordered a bipartisan review of the protective agency after a gunman attempted to assassinate Trump at his campaign rally by firing from a nearby rooftop.

The gunman, Thomas Matthew Crooks, fired eight shots at the rally, killing one man and leaving Trump with a bloody ear. The Secret Service shot and killed Crooks.

On Thursday, the panel called for “a mandate that all outdoor events are observed by overhead technology.”

Another gunman was spotted near the former president outside of the Trump International Golf Course in Palm Beach, Florida in September.

Police arrested him after noticing the tip of a rifle poking through shrubbery a few hundred yards away from Trump who was inside the golf course.