The extension in question was designed by the London–based firm of Ahrends, Burton and Koralek, and was the winner of a competition held in 1982.
Prince Charles's remark was part of a speech he gave on 30 May 1984, as part of a Royal Gala Evening at Hampton Court Palace to celebrate the 150th anniversary of the Royal Institute of British Architects (RIBA). It was so widely reported that the phrase "monstrous carbuncle" quickly became a byword for the worst excesses of modern architecture. And yet, Prince Charles wasn't objecting to modern architecture in general, or even this individual design, per se; his point was that this particular design was not in keeping with its surroundings – the more traditional buildings that had previously adorned Trafalgar Square, including the National Gallery itself.
However it was perceived, the remark hit home and the proposal was dropped.
One of the conditions of the 1982 competition was that the new wing had to include commercial offices as well as public gallery space. However, in 1985 it became possible to devote the extension entirely to the Gallery's uses, due to a donation of almost £50 million from Lord (John) Sainsbury and his brothers Simon and Sir Tim Sainsbury. A closed competition was held, and (reports Wikipedia), "the schemes produced were noticeably more restrained than in the earlier competition." The successful design was the one submitted by Robert Venturi, Denise Scott Brown and Associates; construction began in 1988, and the "postmodernist" Sainsbury Wing was completed in 1991. It was Grade I listed in 2018.
© Haydn Thompson 2021