Britain's Guardian Media Group (GMG) is in exclusive talks to sell The Observer, the world's oldest Sunday newspaper, to startup Tortoise Media, the companies said on Tuesday.

Tortoise said negotiations include a commitment from it to invest more than 25 million pounds ($33 million) over the next five years in the editorial and commercial renewal of the title.

First published in 1791, the centre-left Observer is one of Britain's best-known newspapers and once carried the journalism of George Orwell. It became a sister title to the Daily Guardian newspaper in 1993 after its acquisition by GMG.

"It provides a chance to build The Observer's future position with a significant investment and allow The Guardian to focus on its growth strategy to be more global, more digital and more reader-funded," GMG CEO Anna Bateson said in a statement.

Tortoise was founded in 2019 by James Harding, a former executive at the Times newspaper and BBC News.

"We think The Observer is one of the greatest names in news. We believe passionately in its future – both in print and digital," Harding said.

The Observer, like the rest of the newspaper industry, has suffered a decline in print sales. Its circulation was 136,656 copies in 2021, before GMG stopped publishing ABC sales data.

Its online content is closely integrated with the Guardian's.

Tortoise said it would continue to publish The Observer on a Sunday and build the digital Observer, combining it with Tortoise's podcasts, newsletters and live events.

"Like its many, many loyal readers, we admire the strength and heart of The Observer's reporting, we prize its original, unbiddable thinking and we love it for its passions: food, music, film and art," Harding said.

"George Orwell described The Observer as 'the enemy of nonsense'; we’re excited to show readers, old and new, that it still is," he added.

GMG's 2023-24 results, published on Tuesday, showed pressure from a slowdown in advertising and the long-term print decline.

Overall revenue fell 2.5% to 257.8 million pounds ($340.6 million), it said, and adjusted cash outflow rose to 36.5 million pounds, from 17.3 million pounds in the previous year.

The company is owned by the Scott Trust, an endowment fund valued at 1.275 billion pounds, the results showed.