Richard Flanagan’s Question 7 has been named winner of the Baillie Gifford prize for nonfiction, making the Australian writer the first person to have won both this award and the Booker prize for fiction.

However, in his acceptance speech, Flanagan said he would not accept the £50,000 prize money until the fund manager shares a plan to reduce its investment in fossil fuel extraction and increase investments in renewables.

He said he would welcome an opportunity to speak with Baillie Gifford’s board, to thank them and to “describe how fossil fuels are destroying our country”. He added that his words should not be seen as criticism of the firm.

Part-memoir, part-novel, part-history, Question 7 charts Flanagan’s attempt to understand his parents and Tasmania, where he is from. It is “just a remarkable book” said the chair of the judges, the journalist Isabel Hilton, describing it as “an astonishingly accomplished meditation on memory, history, trauma, love and death – and an intricately woven exploration of the chains of consequence that frame a life”.

Flanagan was unable to attend the London ceremony to collect the prize in person, as he is on a pre-arranged trek in the Tasmanian rainforest. He delivered his acceptance speech via a pre-recorded video.

Baillie Gifford, which has sponsored the prize since 2016, has come under fire in recent years because of its investments in fossil fuels and companies linked to Israel. Earlier this year, literary festival boycotts organised by campaign group Fossil Free Books led to the termination of partnerships between Baillie Gifford and nine festivals.

When it came to choosing the winner, there was apparently “no dissent” between Hilton and her fellow judges, the investigative journalist Heather Brooke, New Scientist’s comment and culture editor, Alison Flood, Prospect’s culture editor, Peter Hoskin, the critic Tomiwa Owolade and the author and restaurant critic Chitra Ramaswamy. Though there was plenty of “vigorous discussion”, the chair said that “oddly, the book that was almost least discussed throughout this was Question 7”, because its merit was apparent to every judge. “Whatever you were looking for in nonfiction, there were elements of it in Question 7.”

The book, which contains an account of a near-death experience the author had, has been described as “unclassifiable”. As well as being chosen for this prize, it has been shortlisted for France’s Prix Femina Étranger, a prize for novels translated into French. When asked about this in a recent Observer interview, Flanagan said he was “delighted” that his book is up for both awards, adding that “labels are for jam jars”.

The Booker and the Baillie Gifford are widely regarded as the UK’s most prestigious literary prizes, for fiction and nonfiction respectively. Flanagan won the Booker in 2014, for his story of a Tasmanian doctor who becomes a Japanese prisoner of war, The Narrow Road to the Deep North. The Baillie Gifford prize (formerly known as the Samuel Johnson prize) has been running since 1999, but this is the first time a former Booker winner has won.

Flanagan was shortlisted along with another writer primarily known for his fiction – Pulitzer-winning novelist Viet Thanh Nguyen, whose memoir A Man of Two Faces was in the running. Also in contention were The Story of a Heart by Rachel Clarke, Nuclear War by Annie Jacobsen, Wild Thing by Sue Prideaux and Revolusi by David Van Reybrouck, translated by David Colmer and David McKay. Each of the shortlisted authors will receive £5,000.

Speaking at the ceremony, Baillie Gifford’s partner Peter Singlehurst said that with the support of the literary community “we would dearly love to continue sponsoring this magnificent prize”.

Hilton said she asked the other judges at the beginning of the process if they had any doubts about whether they should support the prize given the controversy around Baillie Gifford’s sponsorship.

“None of them did, and nor did I, frankly. So we got that conversation over with at the start, and then we concentrated on the books,” she said, pointing out that last year’s winner, Fire Weather, by John Vaillant, was a book about the climate crisis. “These are serious books that need serious attention, and the Baillie Gifford prize helps them to get that attention,” she said.

Organisers said when this year’s shortlist was announced that two authors had asked to withdraw their books from consideration at the time when books were being submitted, with one explicitly stating Baillie Gifford’s sponsorship as the reason.

Three hundred and forty-nine titles published in the UK between 1 November 2023 and 31 October 2024 were considered for this year’s prize. Previous winners have included Empire of Pain by Patrick Radden Keefe and Super-Infinite by Katherine Rundell.

• Question 7 by Richard Flanagan is published by Chatto & Windus (£18.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply

• This article was amended on 20 November 2024 to clarify that that the authors who withdrew their books did so at the beginning of the process, not when the shortlist was being decided.