There’s a mouse in the house — along with a long-lost 20th-century masterpiece.

An art historian was watching the movie “Stuart Little” with his daughter — and spotted in the background a painting long believed lost, it was reported Friday.

“I could not believe my eyes when I saw Bereny’s long-lost masterpiece on the wall behind Hugh Laurie,” said Gergely Barki, a researcher at the Hungarian National Gallery in Budapest who made the discovery on Christmas in 2009.

“Sleeping Lady with Black Vase,” an avant-garde work by Hungarian painter Robert Bereny, was last seen in public in 1928 — and wound up as a prop in the 1999 children’s movie, starring Michael J. Fox (as the voice of the titular mouse), Geena Davis, Laurie and Jonathan Lipnicki.

“A researcher can never take his eyes off the job, even when watching Christmas movies at home,” Barki, who is writing a biography on Bereny, told Agence France-Presse.

In the film, the painting hangs over the fireplace in the Little family’s New York City apartment.

“It was not just on screen for one second but in several scenes of the film, so I knew I was not dreaming. It was a very happy moment,” said Barki, 43, who knew the work from a faded photo taken in 1928.

“I started to write e-mails to everyone involved in the film.”

He eventually tracked down a set-designer assistant, who, unaware of the piece’s significance, bought it as a prop for $500 at an antique shop in Pasadena, Calif.

“She had snapped it up for next to nothing,” Barki said.

She liked it so much she bought it from the studio for herself.

“It was hanging on her bedroom wall in Washington,” Barki said. “Within a year, I had a chance to visit her and see the painting and tell her everything about the painter. She was very surprised.”

Bereny was part The Eight, a group of the avant-garde Hungarian painters. In 1920, he fled to Berlin where he had a love affair with the actress Marlene Dietrich.

Barki guessed the piece’s original buyer was Jewish and left Hungary before or during World War II.

The film assistant sold the painting to an art collector, who will auction it Dec. 13 in Budapest with a starting price of $110,000.

While Barki hasn’t profited from the work’s rediscovery, his book on the artist will benefit.

“It means that I can make a more complete publication of [Bereny’s] oeuvre catalog,” he said.

With Post Wire Services