The owner of the Guardian has confirmed it is in talks to sell the Observer, the world’s oldest Sunday newspaper, to Tortoise Media.

Tortoise has approached Guardian Media Group (GMG) with an offer to invest around £25m over the next five years on the “editorial and commercial renewal” of the Observer.

Tortoise was launched five years ago by James Harding, a former BBC News chief and a former editor of the Times newspaper.

The Guardian reported that the title will remain a seven-day-a-week digital operation regardless of the outcome of negotiations with Tortoise about the Observer.

Observer staff were told that the investment would “help to safeguard its future” as a standalone product.

GMG is not actively trying to sell the Observer, but it is examining the Tortoise proposal to see if it is viable.

Founded in 1791, the Observer is the world’s oldest Sunday newspaper, with a staff of around 70.

“We believe passionately in its future – both in print and digital,” said Harding, who is also editor of Tortoise.

He added: “George Orwell described the Observer as ‘the enemy of nonsense’. We’re excited to show readers, old and new, that it still is.”

Orwell wrote for the newspaper during World War Two and afterwards up until 1948.

Another contributor was Kim Philby, the former MI6 officer and Soviet spy, who had been working for the Russians since the early 1930s.

Like most newspapers, the Observer’s print circulation had been in steadily falling until 2021, when it stopped publishing audited figures. At that point it was selling around 136,000 copies a week.

Harding launched Tortoise with the former US ambassador to the UK Matthew Barzun. It has a brief to provide “slow news” – not chasing breaking stories but rather looking at what drives trends.

It publishes a news website, podcasts and runs live discussions called “Think-ins”.

The business made an operating loss of £4.6m in 2022, the latest year for which accounts are available, with a turnover of £6.2m.

Its financial backers include David Thomson, chair of the media business Thomson Reuters, the tech investor Saul Klein, the investment firm Lansdowne Partners, banker Bernie Mensah and Nando’s executive Leslie Perlman.