The Guardian is reportedly in talks to sell its Sunday paper The Observer to investigative podcast firm Tortoise Media, which is offering to invest £25 million into the title over the next five years.

In an announcement made to staff, Guardian Media Group revealed that it was in the middle of “excusive negotiations” with Tortoise Media after the business approached it with an offer “significant enough to look at in more detail”.

Observer staff have been told that the investment would represent a concrete plan to “safeguard its future” as a standalone title, in the face of steadily falling circulation numbers.

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Founded by former BBC News chief and Times editor James Harding five years ago, Tortoise Media publishes a news website, podcasts and live discussions known as ‘Think ins’.

Its big-name backers include Thomson Reuters chairman David Thomson, tech investor Saul Klein, investment firm Lansdowne Partners, banker Bernie Mensah and Nando’s executive Leslie Perlman.

On the Observer, Harding said: “We believe passionately in its future – both in print and digital. George Orwell described the Observer as ‘the enemy of nonsense’. We’re excited to show readers, old and new, that it still is.”