As family heirlooms go, it doesn't get much better than a pair of Rembrandt portraits that the world had forgotten about for 200 years.

A British family "rather casually enjoyed" the oil paintings by the 17th century Dutch master until an expert from Christie's auction house spotted them.

Now the last pair of Rembrandt portraits to remain in private hands are set to fetch between £5 million and £8 million when they go under the hammer at Christie's in London next month.

"I first encountered these pictures a few years ago on a routine valuation and was stopped in my tracks," Henry Pettifer, International Deputy Chairman of Old Master Paintings at Christie’s, told AFP as the paintings went on display in Amsterdam.

"I was really staggered to discover that the pictures had never really been researched and never been addressed in any of the literature on Rembrandt over the course of 200 years."

The 20-centimetre high (eight-inch) oval portraits, believed to date from 1635, depict an elderly plumber named Jan Willemsz van der Pluym and his wife Jaapgen Carels.

The couple, painted in an unusually intimate style for Rembrandt, were friends of the artist's family and hailed from his hometown of Leiden in the Netherlands.

- 'Casually enjoying' -

An ancestor of the current owners bought the paintings at auction at Christie's in 1824, where they were listed as Rembrandts, and they have remained in the same collection ever since. 

"They've been sitting quietly and enjoyed by the owner's family over the course of two centuries... rather casually enjoying them very much," said Pettifer.

After spotting them, "forensic" work began on verifying that they were genuine.

"Essentially the pictures were unknown, initially to be treated with a great deal of caution, but obviously needed to be examined and researched in great detail," he said.

The auction house enlisted help from art experts, including from Amsterdam's famed Rijksmuseum.

"We were fortunate enough that they had their scientific team really very carefully look into them for nearly two years," Christie's expert Manja Rottink told AFP.

Experts looked at the line of ownership of the paintings, which are mentioned in an inventory of the oldest daughter of the portrait subjects, said Rottink, the senior international specialist of old master paintings at Christie’s.

- 'Very exciting' -

They also checked Rembrandt's signatures, including whether they had been done at the time in wet paint, and comparing the artistic style to his other works.

"The conclusion was that indeed they are by the artist... it was sort of overwhelming how enthusiastic everyone was about this find," she said.

As the smallest known portraits by Rembrandt, they have also shed new light on the style of the artist, who at the time was better known for much larger portraits commissioned by wealthy families.

"These are something slightly different, something much more intimate," said Pettifer.

The newly discovered Rembrandts, which Christie's describes as "one of the most exciting discoveries we have made in the Old Masters field in recent years", have also been on tour in New York and London, where they will go under the hammer on July 6.

While it remains to be seen whether the buyer will be private or a museum, one thing about the auction is certain.

"They've been intact all their lives so we're selling them together," said Pettifer.

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