The fresh police investigation into Prince Andrew’s alarming ties to late pedophile Jeffrey Epstein had started months before 20 officers finally arrested him in a morning swoop on his birthday.

A detective chief inspector from London’s Metropolitan police had contacted the FBI last November asking for help investigating Andrew Mountbatten-Windsor’s links to Epstein, documents released in the latest Epstein files drop show.

At the time, Scotland Yard was digging into allegations that Andrew had asked his bodyguard to dig up details on someone – thought to be his sex accuser Virginia Giuffre – as well as UK airport logs tied to Epstein’s trafficking, the email shows.

“The UK media also appear to be in possession of certain material from the Oversight Committee, including flight manifests from when JE arrived in the UK with other passengers and further allegations around trafficking etc,” the cop said.

It was not immediately clear how the FBI responded to the UK request.

But investigations appear to have ramped up when the Department of Justice released millions of new files — including docs that appeared to show Andrew sending confidential documents to Epstein.

Andrew was arrested in the shocking raid Thursday, which was his 66th birthday.

“He was expecting to spend the day with seven dogs, instead he had 15 coppers at his door,” a source told the Sun, using UK slang for cops.

The officers drove around 130 miles from London to Andrew’s royal estate in Sandringham as part of a top-secret operation, according to the Telegraph.

“The team would have simply been told to report to the station in the early hours,” the ex-cop told the UK paper of the secrecy over the historic raid, the first against a UK royal in more than 400 years.

“They may have even been told to hand over their mobile phones before being given the final briefing, setting out where they were going and what they were doing.”

After the officers arrived around 8 a.m., Andrew would have been informed of his right to remain silent, as is procedure when someone is arrested under the UK’s Police and Criminal Evidence Act 1984.

“You do not have to say anything. But it may harm your defense if you do not mention when questioned something which you later rely on in court. Anything you do say may be given in evidence,” arresting officers often say.

Andrew was then whisked to a police station in Aylsham and would’ve been told to hand over possessions such as his cell phone and jewelry.

He was told of his rights to a lawyer, fingerprinted and had a mugshot taken, the Sun said. Andrew was not handcuffed, but it’s not known if he was hurled into a cell.

While Andrew was in custody, searches were carried out at Sandringham and his former home, Royal Lodge in Windsor. Andrew was formally turfed out of Royal Lodge earlier this month.

A haggard-looking Andrew left the station at around 7.30 p.m. — more than 11 hours after he was arrested.

Searches at Sandringham finished Thursday night, but investigations at Royal Lodge continued Friday.

King Charles III, Andrew’s brother, said the law must “take its course” following Andrew’s arrest. He was not told about the arrest beforehand.

National Police Chiefs Council officials informed the UK Home Office that the arrest was taking place around 30 minutes before it happened.

Andrew,  who was stripped of his royal titles last year, has always denied wrongdoing in relation to Epstein.

He could face a maximum sentence of life in prison if convicted.

The arrest was strictly related to the allegations of misconduct in public office. If Andrew ends up on trial, it will be known as Rex — Latin for “the king” — versus Andrew Mountbatten-Windsor.