In the latter half of the 19th century the locale of South Kensington was developed as "Albertopolis", a cultural and scientific quarter. Three major national museums are there: the Victoria and Albert Museum (for the applied arts), the Natural History Museum and the Science Museum. The National Portrait Gallery was founded in 1856 to house depictions of figures from British history; its holdings now comprise the world's most extensive collection of portraits. The national gallery of British art is at Tate Britain, originally established as an annexe of the National Gallery in 1897. The Tate Gallery, as it was formerly known, also became a major centre for modern art; in 2000 this collection moved to Tate Modern, a new gallery housed in the former Bankside Power Station.

Answer this question, if possible (if impossible, reply "unanswerable"): London's Tate Britain and Tate Modern galleries were formerly one entity known as what?
The Tate Gallery