The picture from 1914-1917 had been purchased by the European vendor at Christie's New York in May 1997 for $3.5m (£2.16m) hammer. Here the bidding opened at £4.8m but, after a Russian underbidder eventually dropped out, it was knocked down to an anonymous phone buyer.
The 6ft 7in x 3ft 4in (2.01 x 1.01m) oil on canvas had been stamped with the artist's signature which made it less commercial than a fully signed painting. Iris mauves was one of 20 views that Monet painted of irises on the banks of the lily pond and may well have been conceived as part of the decorative ensemble known as the Grandes decorations that Monet spent a decade working on and revising.
Elsewhere in the sale, Tête by Pablo Picasso (1881-1973) was estimated at £4.8m-6.5m and had been guaranteed by the auctioneers. It got away under estimate at £3.9m to a lone phone bidder.
It had previously sold for $6.8m (£4.46m) including premium at Christie's New York in May 2010 where it had been acquired by the vendor.
Not deemed in premier-league of works by the artist, it dated from 1969 and depicted a masculine portrait - an alter ego figure which occurs in a number of Picasso's late works. The highest prices for the artist have come for female portraits from the 1930s, includingLes femmes d'Alger (Version 'O') which sold at $160m (£108.1m) at Christie's New York in May and set a record for any work of art sold at auction.
When it was first exhibited in 1970, it was shown at the Palais des Papes in Avignon alongside 167 oils and 45 drawings which the artist had completed in the year leading up to his 89th birthday.
The overall total at Christie's was £71.5m (including premium) against a presale estimate of £58.7m-85.2m. Forty-two of the 50 lots found buyers on the night (84%).
The Impressionist & Modern Art auction series continues tomorrow night with Sotheby's evening sale estimated to raise £140m-203m.
The buyer's premium was 25/20/12%.