LONDON (REUTERS).- Rivals Christie's and Sotheby's are expecting to hold their biggest ever London sales later this month, reflecting growing confidence in the art market boom.

Christie's announced on Thursday it was offering a 1906 Monet water-lily painting worth an estimated 30 to 40 million pounds ($44-59 million), the same price as a Blue Period portrait by Pablo Picasso.

The pair, plus 61 other works on offer, are expected to take the tally on June 23 to 164-231 million pounds, which, if realized, would be well above the London record of 147 million pounds set at Sotheby's in February.

"We are witnessing a great willingness from clients to consign works of art of the highest quality," said Giovanna Bertazzoni, head of impressionist and modern art at Christie's.

Sotheby's said it expected to sell art worth 101-148 million pounds at its equivalent sale on June 22, its highest pre-sale estimate in London.

Its auction centerpiece is a painting by Edouard Manet valued at 20-30 million pounds, one of just two self-portraits by the artist and the only one in private hands.

Records Tumble
Both of the world's leading auctioneers have set records in recent months, starting with Sotheby's in February when a Giacometti statue went under the hammer for $104.3 million, the world auction record for a work of art.

Three months later, Christie's topped that with a Picasso which fetched $106.5 million in New York.

The top European auction total stands at 183 million pounds for the impressionist and modern art section of the private Yves Saint Laurent collection.

With records tumbling, it comes as little surprise that owners of the finest art are willing to offer it up for sale.

Uncertainty over the state of the broader global economy has failed to dampen the mood in sales rooms, experts say, with only a handful of super-wealthy individuals or institutions needed to drive values higher.

A relatively short-lived slump in the art market was caused as much by sellers drying up as by buyers no longer wanting to pay out large sums for paintings and sculptures.

Christie's described demand for the rarest works of art as "fierce," coming from Russia, China and the Middle East as well as the more traditional markets of Europe and North America.

"Nympheas," the work by Monet to be offered by Christie's, was shown at the famous 1909 exhibition in Paris where the artist's studies of the effects of light in his garden in Giverny won critical acclaim.