The shocking truth behind MICHELLE OBAMA‘s $500,000 a day European "vacation".

Furious that her husband Barack Obama invited Oprah Winfrey to his Chicago birthday bash, first lady Michelle blew nearly half a million bucks on a shopping spree in Spain!

Now the first couple are facing a huge backlash because her extravagant overseas vacation will cost U.S. taxpayers an estimated $75,000 a day!

"With the American economy in shambles, Michelle took off on a lavish holiday all because she was angry that Barack had invited Oprah to his private 49th birthday party," a top Washington, D.C., insider told The ENQUIRER.

"Michelle decided to punish Barack, so she bought expensive clothing and designer dresses plus jewelry while staying in a luxury resort – not realizing how the public would react back home. Her selfish spending spree really backfired!"

Michelle, several friends and her 9-year-old daughter Sasha stayed in Marbella at the Hotel Villa Padierna, a ritzy resort with two golf courses, a spa and views of the surrounding mountains, where its royal villa costs $6,500 a night. While the first family picked up their personal expenses, taxpayers will foot the bill for the first lady’s transportation – at least $146,000 for a U.S. Air Force 757 – and security detail, which included three shifts of uniformed and plain-clothes Secret Service agents.

Michelle and her entourage spent five days in Spain – at an estimated cost to taxpayers of $375,000.

And the first family was treated like royalty as the first lady also dressed the part – wearing a flashy one-shouldered top by designer Jean Paul Gaultier during a shopping jaunt.

"Michelle also bought an expensive necklace and bracelet in Marbella," said the insider. "While she wanted to make a side trip to the millionaire’s playground of Ibiza, she had a personal shopper bring designer gowns and clothing, swimsuits and other trinkets to her instead."

Meanwhile, the American press roasted Michelle for her luxe getaway, with one newspaper columnist calling her "a modern-day Marie Antoinette" for vacationing while the economy lost 131,000 jobs.

Michelle responded by saying that she made the trip to spend time with a good friend who recently lost her father.

But those close to the first couple say the real reason for the vacation was her anger when she heard that Barack had invited Oprah to his birthday celebration.

With his wife in Spain, President Obama celebrated his birthday with Oprah and other friends on Aug. 4 in Chicago.

According to an insider, Michelle feels Oprah has tried to co-opt her position as the president’s top adviser, and she’s managed to shut out the talk-show queen from most White House events.

"Michelle is furious that Barack went behind her back and invited Oprah," said the insider. "But he’s well aware that Oprah helped his campaign immensely by endorsing him for president, and he knows that he’s going to need her again.

"He hates having Michelle mad at him, but he needs Oprah —  on his side."