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 meeting by the boards of the companies who own it - Scott Trust and the Guardian Media Group.

The move followed a 48-hour strike this week by journalists at the paper and a 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 news.

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

The company 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 paper.

Speaking after the sale was announced, Harding said he was "honoured and excited 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 give it a new lease of life as a powerful, progressive voice in the world".

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 new owner.

National Union of Journalists general secretary-elect, Laura Davison, said the 233-year-old newspaper "holds a unique and important place in public life and our members care about the next chapter in its history".

Katharine Viner, editor-in-chief of Guardian News and Media, said: "I recognise how unsettling this period has been for Observer staff but we're confident we have agreed on the best possible way forward for the title's journalists, its readers and the future of both the Observer and the Guardian.

"It is a model that will see investment in journalism and journalists, enshrines the Scott Trust's values in the Observer's future, and protects the Observer and Guardian's ability to continue to produce trusted liberal journalism."

The Guardian Media Group 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 take 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 Group, said: "This investment will preserve the Observer's 233-year legacy and protect the paper's future, ensuring it can continue producing exceptional liberal journalism, online and in print, for years to come. Underpinning it all will be a continued commitment to promoting a free press and maintaining editorial independence.

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