WASHINGTON — A State Department whistleblower has accused high-ranking staff of a massive coverup — including keeping a lid on findings that members of then-Secretary Hillary Clinton’s security detail and the Belgian ambassador solicited prostitutes.

A chief investigator for the agency’s inspector general wrote a memo outlining eight cases that were derailed by senior officials, including one instance of interference by Clinton’s chief of staff, Cheryl Mills.

Any mention of the cases was removed from an IG report about problems within the Bureau of Diplomatic Security (DS), which provides protection and investigates crimes involving any State Department workers overseas.

“It’s a coverup,” declared Cary Schulman, a lawyer representing the whistleblower, former State Department IG senior investigator Aurelia Fedenisn.“The whole agency is impaired.

“Undue influence . . . is coming from political appointees. It’s coming from above the criminal- investigation unit,” added Schulman, whose client provided the document with the revelations.

Some of the revelations were first reported by CBS News.

Among the bombshell findings:

* A DS agent was called off a case against US Ambassador to Belgium Howard Gutman over claims that he solicited prostitutes, including minors.

“The agent began his investigation and had determined that the ambassador routinely ditched his protective security detail in order to solicit sexual favors from both prostitutes and minor children,” says the memo.

“The ambassador’s protective detail and the embassy’s surveillance detection team . . . were well aware of the behavior.”

Undersecretary of State for Management Patrick Kennedy ordered the investigation ceased, and the ambassador remains in place, according to the memo.

Gutman was a big Democratic donor before taking the post, having raised $500,000 for President Obama’s 2008 campaign and helping finance his inaugural.

* At least seven agents in Clinton’s security detail hired prostitutes while traveling with her in various countries, including Russia and Colombia.

Investigators called the use of prostitutes by Clinton’s security agents “endemic.”

The liaisons with prostitutes allegedly occurred in the same hotel where Clinton slept, according to sources familiar with the incident.

But the agents involved got little more than a wrist-slap. Three were removed from the security detail, given one-day suspensions and reassigned.

“No further investigations have occurred regarding the remaining four, despite the possibility of counterintelligence issues,” says the memo.

According to the memo, members of the Special Investigations Division (SID) approached the agent who was probing “and reportedly told him to shut down the four investigations.”

The incident in Colombia occurred prior to the scandal involving President Obama’s Secret Service detail in Cartagena, Colombia, but the State Department’s misconduct did not come to light until now.

The memo references a “rumor” that after the Secret Service hooker scandal broke, Clinton asked the agent-in-charge of her security detail whether any “similar activities” had happened. “The response was: ‘No,’ ” according to the memo.

* The case in which Clinton enforcer Mills allegedly intervened centered upon Brett McGurk, Obama’s nominee to be US ambassador to Iraq.

McGurk’s expected nomination fell apart after a computer hack exposed his racy e-mails and an extramarital affair with Wall Street Journal reporter Gina Chon.

According to the memo, the SID “never interviewed McGurk, allegedly because Cheryl Mills from the Secretary’s office interceded.”

“Without that interview, SID has been unable to close the case,” the memo concludes.

The memo cites an e-mail from Mills showing her agreeing to a course of action, “but then reneging and advising McGurk to withdraw his name from consideration for the ambassadorship.”

McGurk withdrew right before consideration by the Senate Foreign Relations Committee.

* The document states that a security officer stationed in Beirut, Chuck Lisenbee, allegedly engaged in sexual assaults against local guards.

A diplomatic security higher-up called the investigation a “witch hunt” and gave agents only three days to look into the charge, the memo says.

State Department spokeswoman Jen Psaki said the agency was conducting internal investigations of all the cases that have come to light, and wouldn’t ignore serious charges against top officials.

“We take allegations of misconduct seriously and we investigate thoroughly,” she said.

“The notion that we would not vigorously pursue criminal misconduct in a case, in any case is preposterous.

“And we’ve put individuals behind bars for criminal behavior,” she said.

“Ambassadors would be no exception.”

Psaki said State has brought in outside officials with law-enforcement backgrounds to assist in the investigation. The agency that conducts internal investigations is bringing in “experienced law-enforcement officers” to conduct a review, she said.

Officials at The State Department declined to comment on the specific allegations.

Gutman said today in a statement,”I am angered and saddened by the baseless allegations that have appeared in the press and to watch the four years I have proudly served in Belgium smeared is devastating.

“I live on a beautiful park in Brussels that you walk through to get to many locations and at no point have I ever engaged in any improper activity.”

Still, even the IG, which is supposed to be independent, bowed to pressure to remove mention of these embarrassing cases, according to internal documents.

At a December 2012 meeting to prepare the report, Assistant Secretary of State for Diplomatic Security Eric Boswell said he was “stunned” by the findings, and requested that the cases should be omitted.

“He proposed that the subject ‘should be withheld’ from the inspection report until INV’s process determines if ‘there is something there,’” according to notes from the meeting.

“Boswell said putting the subject in the report would ‘generally damage [Department of State],’ would ‘probably damage the Department,’ and would be used by ‘every defense lawyer around,’” according to the notes.

They further said that he wanted to wait to see if something “came of it.”

Fedenisn, the whistleblower, did not take the notes but was charged with keeping them, according to her lawyer.

The draft report, marked “Sensitive But Unclassified,” cites several examples of undue influence “from the top floor of the department, raising serious concerns about the quality and integrity” of investigations.

That statement was removed from the final report issued March 15.

The final report also removed mention of “an ambassador accused of pedophilia and another such senior official had [Diplomatic Security] stop an investigation of an ambassador designate.”

“Hindering such cases can result in counterintelligence vulnerabilities and can allow exploitive criminals to continue their activities,” said the draft report.

“Moreover, the interventions frustrate, even demoralize,” dedicated agents, it went on.

Fedenisn’s lawyer, Schulman, said that his client came forward after a long career as a State Department special agent and IG investigator because she was “thrust into this.”

“She did it because it was the right thing to do,” he said. “Aurelia courageously has come forward with nothing to gain at all . . . Nobody gives you a prize for this.”

He said that Fedenisn, who retired in December, has been threatened by State Department officials with criminal charges.

“She wants to go back to her life,” said Schulman. “She’s not looking for fame.”