They were billionaires but lived like paupers in a $100 million London mansion.

They were one of Europe’s glamour couples, friends of Prince Charles, but retreated from ornate ballrooms and charity galas to a filthy drug den in their home.

The secret life of Hans and Eva Rausing was exposed Monday when British police busted him for drug possession — then found the remains of his addict wife in an upstairs bedroom of their home in the Belgravia section.

Investigators suspect Hans, the 49-year-old heir to the $6 billion Tetra Pak food-packaging empire, had been living in a drug-fueled stupor with the body of the US-born Eva, 48, for as long as week.

A friend said that in recent months, the couple dressed shabbily instead of in the tuxedos and gowns they were known for. They both lost weight, turning into gaunt figures who looked much older.

The Rausings also retreated to two rooms of the six-story Georgian townhouse that they turned into a crack den.

They told their servants not to enter the rooms. Police believe Hans Rausing rarely left them in the final days of his wife’s life, except to buy drugs.

“It was total squalor. Really messy. You wouldn’t believe they were billionaires,” a friend told British newspapers. “It shows the effect of drugs. They couldn’t look out for themselves or their house.”

It was a disastrous descent for Eva Rausing and her husband, a member of Britain’s 12th-richest family.

Eva, daughter of a wealthy former Pepsi executive, met Hans Rausing around 1990 — at a drug rehab clinic in California.

Hans had grown up in Sweden, where his grandfather founded the family’s carton enterprise in the 1930s. But he showed no interested in the business.

“He has never had a job in his life,” said Swedish journalist Peter Andersson, who chronicled the family in his book, “Tetra — The History of the Rausing Dynasty.”

“Instead, he went off to India to find himself. He came back from there seriously ravaged by drugs,” Andersson said.

Eva later hinted she got hooked about the same time, the late 1980s, while attending a California college.

“I had a good time — too good, as I dropped out and did not go back to university until the grand old age of 24,” she wrote on MySpace. “Which leaves some troubled years in between.”

But she cleaned up and “became a good girl, if maybe a little boring, got a degree in economics,” she added.

She and Hans married soon after they met, and they began a new life in London.

They had four children — “I suppose they are my greatest achievements,” she wrote — and became known for their philanthropic work, including serving on the boards of anti-drug charities.

Over the years, the Rausings gave tens of millions of dollars to Action on Addiction — but didn’t disclose their own battles, said the group’s CEO, Neil Barton.

“I didn’t want to go there unless she wanted to talk about it,” he said. “I don’t remember ever having that sort of conversation with her, but it was understood that that was partly why she was interested.”

But Eric Carlin, former chief executive of another anti-drug charity, Mentor, said Eva was frank about her addiction.

When a charity trustee told her, “We all agree we don’t like drugs,” she replied: “No. I love drugs. That’s the problem.”

Her secret life was exposed four years ago, when security guards found crack and heroin in her handbag during a visit to the US Embassy on a passport matter.

Police were summoned and found more drugs in her car. That led to a search of the Rausings’ home, which turned up $3,000 worth of pure cocaine and more crack and heroin.

Despite the scandal, Prince Charles stood by her and backed a decision to keep her as a trustee of his Foundation for the Built Environment.

But Eva’s problem got worse.

“She was totally addicted, obviously, and was trying to get off it,” Liz Brewer, a friend, told Sky News.

“The last time I saw her was a couple of months ago,” she added. “She had cut herself off from people for a while.”

She said Eva’s problem had been “pushed under the carpet” for too long. Eva’s circle should have intervened after the embassy incident, she said.

“Had it happened then, maybe both of them would have come to their senses,” she said.

But Eva’s sister, Be Kemeny, said Eva appeared to be OK when they celebrated last Christmas in Barbados.

“I want to know my sister’s last words,” she told Britain’s Daily Telegraph. “I know she would have wanted to say something to me. She’s the bright vivacious one, the friendly one.”

Eva’s parents issued a statement after her death: “During her short lifetime, she made a huge philanthropic impact, supporting a large number of charitable causes, not only financially, but using her own personal experiences.”

“She bravely fought her health issues for many years. The family is devastated at her death and asks to be given privacy at this difficult time.”

Police have yet to declare a cause of death.

Investigators have not been able to question Hans at length because he is suffering drug withdrawal at a medical clinic.

One theory is he was overwhelmed by Eva’s death and simply remained stoned to cope.

He was stopped by police Monday after driving erratically in south London. Police found he had drugs.

Investigators are examining Eva’s MySpace and Facebook postings, as well as back records, e-mails and security video footage of the mansion.

In 2007, she wrote on MySpace, “I fell back into same hole as before and have been there for nearly seven years.”

“I’m still married, amazingly, to a very kind, patient and loyal husband,” she added. “I’m very lucky that he has stuck with me — many would not.”