Sometimes a single building becomes the focus for an architect's endeavours and reputation. For John Bancroft, who has died aged 82, that building was Pimlico school. Not only did Bancroft design and see this striking landmark of the 1960s through to completion, he also waged an unremitting and lonely struggle for more than a decade to save his cherished creation from destruction, to no ultimate avail.

Pimlico was political from the start. A monument to the comprehensive schooling policies of the Inner London Education Authority and the architectural vagaries of the Greater London council, it was imposed in 1967–70 on a razed and open urban block in the heart of Tory Westminster. A little earlier, and a school in a tower block might have faced off against the surrounding stucco terraces. But by the mid-60s the experts knew what children could do in and to lifts. So Bancroft, the GLC's inhouse job architect, opted for a walk-up building of four storeys only, linear and compact, with a stepped section to maximise daylight. The lowest storey was sunk to the levels of the former townhouse basements. Out of this pit, like a creature in a zoo, grew the concrete-and-glass school, glaring at the rectangle of streets all round. Boxy projecting classrooms with canted glazing, supposedly self-cleaning, completed the brutalist effect of provocation.

John Bancroft waged an unremitting struggle for more than a decade to save his cherished creation, to no avail

Unluckily for Bancroft, Pimlico school was out of date when it opened. Educational ideas change fast, and he had been handed an outdated brief. The bigger spaces worked well, but the classrooms were inflexibly shaped and grouped, while the double-height concourse that was the school's heart was never put to full use after the departure of the enthusiastic first headteacher, Ken Green. Worse, the heating and cooling system was rapidly vandalised, and no lasting solution to the extreme solar gain in the classrooms could be found.

Pimlico soon earned itself a reputation, especially in music and drama, but did so despite its remarkable building, not because of it. When Westminster council, casting greedy eyes upon the site, decided in 1995 to redevelop half of it with luxury flats and create a smaller school on the other half under a PFI scheme, the idea proved hard to combat. Bancroft, by then long retired but always a doughty campaigner, summoned up influential architectural allies and saw the first scheme off, maintaining that simple changes could renew the school. But he was hamstrung by his inability to get Pimlico listed, ministers taking the expedient view that inherent design faults impaired its architectural value. The last remnants of Pimlico school disappeared this year in favour of a faceless substitute.

Bancroft was born in London and brought up in Nottingham. His civil-servant father was an amateur painter and also collected books, a passion which John fully inherited. He started as a draughtsman in a brewery, where someone noticed his talent and persuaded his father to pay for his training at Nottingham University. After national service at Chatham in Kent, he worked for the local borough council there before moving in 1954 to Crawley Development Corporation, in Sussex.

His ambitions took off only when he joined the schools division of the London county council's architects' department in 1957. There, John caused amusement by wearing a smock at the drawing board, but proved his credentials with designs for Elfrida Rathbone (now Haymerle) school, in Peckham, and an extension to Philippa Fawcett college (now Dunraven school), in Streatham. After Pimlico, he was shunted into an administrative role in the housing division, and retired early in 1980.

Though loyal to the public service and collective ethic, Bancroft was at heart an individualist who regarded his calling as a high art with spiritual aims. Critical of most architecture of his day, as early as 1973 he announced, "I am a Victorian at heart." True to his word, he was active in the Victorian Society. Under his leadership a clique called the Dinosaur Five gingered up the GLC in 1979 to oppose a plan by the Natural History Museum to destroy the side galleries of their Grade I-listed building. The campaign's climax was a cake baked in the shape of the museum, from which Spike Milligan cut the threatened galleries before the attendant press. An alternative scheme devised by Bancroft helped save them.

During his retirement he was involved in the restorations of HMS Warrior, then at Hartlepool and now at Portsmouth, and of the SS Great Britain at Bristol. He also designed premises for Howes' Bookshop at Hastings, the source of many acquisitions decking his walls at Haywards Heath in Sussex.

John was a gravel-voiced character with streaks of grit and obstinacy but liberal views and a saving sense of humour. He is survived by his fourth wife, Janet, and by a daughter, Sarah, from his second marriage.

• John Bancroft, architect, born 28 October 1928; died 29 August 2011