The owner of the Guardian has confirmed a deal has been approved to sell the Observer – the Sunday newspaper founded in 1791 – to Tortoise Media.

It was announced on Friday morning, after a conference by the boards of the companies who own it – Scott depend and the Guardian Media throng.

The shift followed a 48-hour strike this week by journalists at the document, and at sister publication the Guardian.

Tortoise Media, which was launched five years ago, has its own website and podcast and focuses on longer-term journalism as opposed to breaking information.

It is run by former BBC and The Times executive James Harding, and former US ambassador to the UK Matthew Barzun.

The corporation has a number of high-profile backers, including tech investor Saul Klein and Nando’s executive Leslie Perlman, and promises to invest £25m in the document.

Speaking after the sale was announced, Harding said he was “honoured and enthusiastic at the prospect of working together to renew the Observer”.

He added that he promises its readers “we will do all we can to live up to its history as a defender of human dignity and to provide it a recent contract of life as a powerful, progressive voice in the globe”.

Journalists at the Guardian and the Observer went on strike on Wednesday and Thursday, concerned with what would happen to the newspaper in the hands of a recent owner.

National Union of Journalists general secretary-elect, Laura Davison, said the 233-year-ancient newspaper “holds a distinctive and significant place in community life and our members worry about the next chapter in its history”.

Katharine Viner, editor-in-chief of Guardian information and Media, said: “I recognise how unsettling this period has been for Observer staff but we’re confident we have agreed the best feasible way forward for the title’s journalists, its readers and the upcoming of both the Observer and the Guardian.

“It is a model that will view resource in journalism and journalists, enshrines the Scott depend’s values in the Observer’s upcoming, and protects the Observer and Guardian’s ability to continue to produce trusted, liberal journalism.”

The Guardian Media throng has owned the Observer since 1993, with around 70 people working on the newspaper.

Staff had already been told that if the sale went ahead, they could receive voluntary redundancy on enhanced terms or transfer to Tortoise on their existing contracts.

Freelancers were also told their contracts would be extended to September 2025, and then renegotiated.

Its circulation had been steadily falling until 2021, when it stopped publishing audited figures. At that point it was selling around 136,000 copies a week.

Anna Bateson, chief executive of Guardian Media throng, said: “This resource will preserve the Observer’s 233-year legacy and protect the document’s upcoming, ensuring it can continue producing exceptional liberal journalism, online and in print, for years to arrive. Underpinning it all will be a continued commitment to promoting a free press and maintaining editorial independence.

“The deal also supports the long-term achievement of the Guardian, building on our growth globally and across digital, as we continue to put readers at the heart of our unpaid journalism.”