Prince William's wife Catherine made her first solo public engagement on Wednesday while her husband is away in the Falkland Islands on a Royal Air Force mission.

The former Kate Middleton, 30, visited a major exhibition of paintings by the late British artist Lucian Freud at the National Portrait Gallery in London, on the eve of its opening.

Catherine, a patron of the gallery and a former art history student at St Andrew's University in Scotland where she met Prince William, wore a grey tweed coat-dress as she visited the exhibition.

William is on a six-week tour of duty as a search and rescue helicopter pilot in the Falkland Islands -- a deployment that Argentina, which claims the British-ruled archipelago as its own, has slammed as "a provocation".

The visit of the woman whose official title is Duchess of Cambridge came as St James's Palace confirmed she is to pose for a portrait by an artist who is yet to be chosen.

"The Duchess is happy to do it some time in the future," a palace spokesman said. "There haven't been any decisions about who the artist will be, this is being discussed."

The Freud exhibition, which opens to the public on Thursday, includes "Benefits Supervisor Sleeping", a 1995 canvas which sold for $33.6 million in New York in 2008 -- a world record for a living artist.

Freud died in July at the age of 88.