The National Portrait Gallery has set off a legal fight against Wikipedia.
The gallery, situated near Trafalgar Square in London, said the online user-generated encyclopaedia had uploaded more than 3,000 copyright protected images from the gallery's own website.
High resolution images were taken "without permission", potentially hurting an important source of revenue, the gallery alleged.
"In March 2009 over 3,000 high-resolution files were appropriated from the National Portrait Gallery website and published on Wikipedia without permission," a spokesperson at the gallery told the Evening Standard newspaper.
"The gallery is very concerned that potential loss of licensing income from the high-resolution files threatens its ability to reinvest in its digitisation programme and so make further images available. ... Digitisation involves huge costs including research, cataloguing, conservation and highly-skilled photography."
National Portrait Gallery said it had attempted to contact Wikipedia, asking it to discuss the issue. "To date, Wikipedia has not responded to our requests to discuss the issue and so the National Portrait Gallery has been obliged to issue a lawyer's letter," the spokesperson told the newspaper.
But the gallery said it has had difficulty tracking down anyone who can address the issue at Wikipedia.
It could be a complicated battle in legal terms, the Evening Standard reported. In British law photographs are covered by copyright, but in the US a photograph of an image in the public domain is not.
One user, who on US website Tech Radar called himself Derrick Coetzee, said he had contributed to the images being uploaded. But he denied he was employed by Wikipedia, saying he was a "volunteer contributor". Wikipedia has not yet commented on the issue.
The gallery has so far digitised 60,000 of its paintings under a £1 million scheme.