FBI surveillance of former Trump campaign advisor Carter Page cost him business, income and even his girlfriend.

Page tells The Post that during the media barrage he faced in late 2016, he visited his girlfriend at her London flat, where she was “freaking out with the fake news about me.”

“Talking with her later in the evening after dinner, she told me that she didn’t want me staying there anymore, and that our relationship was over.

“So late that night,” Page continued, “I booked a last-minute hotel reservation as part of this early chapter of the redefinition of my life.”

Page believes the FBI’s mole, professor Stefan Halper, was secretly spying on him as part of a “politically motivated” investigation of Team Trump, using fake sympathy to gain his trust — all while fishing for dirt on Page’s ties to Russia, where he’d worked as an energy consultant.

“I wouldn’t be surprised if it turned out to be a trap,” Page said.

Halper contacted Page in early July 2016, weeks before the FBI claims it first opened an investigation into the Trump campaign’s ties to Russia.

The timing is significant because if Halper was being used as an informant before a case had been officially opened, it raises the possibility the Trump campaign was spied on for political — not law enforcement — reasons.

Page believes that’s exactly what happened, and agrees with the president’s characterization of the scandal as “Spygate.”

“It is particularly appropriate given recent developments,” the former campaign aide said.

Page said while he was under surveillance, Halper “portrayed himself as someone offering help” to him and the campaign, providing “insights and perspectives” on various foreign policy issues, including relations with Russia.

Page recalled the professor “expressed sympathy about what I had been put through with the defamation and smear campaign led by the DNC and the Clinton campaign,” which underwrote the dossier claiming Page conspired with the Kremlin to swing the election for Trump.

For instance, Halper penned a July 28, 2017 “Dear Carter” e-mail, assuaging him that new White House controversies may have taken pressure off him.

“I must assume this gives you some relief,” Halper wrote, adding that it “would be great to catch up.” Halper signed the note, “Stef.”

Since the breakup of his relationship and the decline of his consulting business, Page has gone on the offensive, sharing his side of the story as Washington politicians battle over the role that Halper, a British-American professor, played while informing on top Trump campaign officials starting in the summer of 2016.

News outlets recently identified Halper as the paid FBI informant who reached out to several Trump officials during the campaign.

Page said the FBI had “no basis” for spying on him or the campaign with individuals like Halper or through electronic means. He described the bureau’s clandestine activities as criminal.

Trump again railed against the Dems on Saturday, tweeting that they had planted spies “all over my campaign.”

Page echoed the president’s description of the scandal as worse than Watergate, in which then-President Richard Nixon directed spying on the Democratic Party in 1972.

“The crimes that have already been committed against President Trump, myself and the entire Trump movement are much worse,” he said.

Page says Halper, who has longstanding ties to the CIA and British intelligence, stayed in touch with him over 14 months, even inviting Page to his farm in Great Falls, Va., not far from CIA headquarters. The pair lost touch in September 2017 — the same time the warrant to monitor Page expired.

The Foreign Intelligence Surveillance Court had issued a warrant to spy on Page in October 2016, renewing it three times. Page was never criminally charged in special counsel Robert Mueller’s investigation into whether the Trump campaign had anything to do with possible Russian interference in the presidential election.

Halper had voiced support for Hillary Clinton. In March 2016, he was quoted as saying, “I believe Clinton would be best for US-UK relations and for relations with the European Union.”

Halper reached out to other Trump campaign aides — George Papadopoulos and Sam Clovis — at the behest of the FBI, according to reports. Halper reportedly also informed on Gen. Michael Flynn, who became Trump’s national security advisor for a short period.

The information Halper developed reportedly has been provided to Mueller.

Halper could not be reached for comment.

The FBI routinely uses confidential informants in counterintelligence investigations, and the deployment of such sources in 2016 does not by itself indicate a political attack on Trump.

However, law enforcement officials say they doubt the FBI had enough of a legal basis to sic Halper on Page or Papadopoulos. They say then-Attorney General Loretta Lynch would have had to sign off on the move.

House Intelligence Committee Chairman Devin Nunes says the FBI never had enough evidence to open a criminal or counterintelligence investigation into the Trump campaign in the first place.

Page, a Poughkeepsie native, volunteered for the Trump campaign in January 2016 on a recommendation from the chair of the New York GOP, Ed Cox. Page, whose office used to be located next to Trump Tower, says he always admired the president as a businessman.