The National Gallery of Canada is defending its decision to auction off a Marc Chagall masterpiece in order to secure a Canadian work of art.

On May 15, Christie's is auctioning The Eiffel Tower, one of only two Chagall oils owned by the gallery. The auction house estimates the painting will fetch between $6 million and $9 million US.

The proceeds will be used to buy an important work of "national heritage," according to the gallery.

"There's a work that's about to leave Canada that we feel should not leave Canada," said Marc Mayer, director and CEO of the National Gallery of Canada.

"It's a more important work of art than the ones that we're proposing to sell."

The Eiffel Tower is one of eight pieces the gallery's board of trustees approved last June to sell or donate to other facilities. It's part of a new "disposition policy" that will see the gallery put more items from its current collection on the block, or send them elsewhere.

'We've exhausted all of our options'

Mayer said the gallery chose to sell Chagall's piece because it was in storage for years and the money it will bring in will allow the gallery to purchase the pricey Canadian work of art. The gallery's annual acquisition budget is $8 million.

"There are about eight PhDs in art history who agreed that we were proceeding properly. These are advisers to our acquisition committee, the curators who identified this work as the right one," he said.

"We've exhausted all of our options to find other ways to acquire this work, but unfortunately this is all that we have left and it is a sacrifice," he said.

Marc Mayer, director and CEO of the National Gallery of Canada, says this was a hard decision. 0:54

Mayer said the gallery is hoping to sell the piece quickly in order to purchase the Canadian piece before it leaves the country.

Mayer said the seller, who approached the gallery, has promised to sell the work to the gallery but does not want to reveal their name to the public or the name of the piece in question.

However, Mayer said the only artist whose work has garnered such a high price is Lawren Harris.

Harris's painting Mountain Forms sold for a record $11.21 million in 2016, making his 1926 piece the most expensive artwork ever to sell at a Canadian auction.