The sale of the Observer newspaper to Tortoise Media has been approved, it was announced on Friday.

The move by the boards of its owner Scott Trust and the Guardian Media Group followed a 48-hour strike this week by journalists on the Observer and the Guardian in protest at the sale.

The Scott Trust said it will invest in Tortoise Media, becoming a key shareholder, taking a seat on both its editorial and commercial boards.

The deal will see £25 million in new investment for the Sunday newspaper, with a commitment to print the Observer on a Sunday and a plan to build it into a digital brand by the boards of the Scott Trust and Guardian Media Group, they announced.

Tortoise Media said it has committed to safeguard journalistic freedom and the editorial independence of the Observer, undertaking to honour the “liberal values and journalistic standards” of the Scott Trust in their editorial code.

It is expected the deal will be signed in the coming days.

Ole Jacob Sunde, who chairs the Scott Trust, said: “We knew we needed the right combination of resources and commitment to build a new platform for the Observer.

“It required an ally to be sufficiently funded, long-term in nature and respect editorial independence and liberal values. I believe we have found this in Tortoise Media. We are looking forward to being part of the next phase in the Observer’s journey.”

Katharine Viner, editor-in-chief of Guardian News & Media, said: “I recognise how unsettling this period has been for Observer staff, but we’re confident we have agreed 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.”

Charles Gurassa, chair of the Guardian Media Group board, said: “This is an important new chapter for the Observer, giving it access to much-needed investment, enabling it to build a long-overdue digital presence and ensuring it the top-level management support and focus necessary for it to flourish. For the Guardian, it means we can double down on our long-term growth strategy, as we continue to expand globally and across digital and other media channels.”

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.”

National Union of Journalists general secretary-elect Laura Davison said: “Members at the Guardian and Observer will be angry and extremely disappointed that the Scott Trust and GMG boards have chosen to approve the deal in principle to sell The Observer to Tortoise Media despite the union’s call for more time to consider this and other options.

“The timing of the decision, before the end of two extremely well-supported days of action, is particularly shabby. The NUJ chapel will be meeting later today to consider their response.”

The two days of strikes this week were supported by a number of politicians and well-known cultural figures including Grayson Perry, Lemn Sissay, Anne Robinson, Michael Rosen and Stewart Lee, and backed by messages of support from stars including Tilda Swinton, Maxine Peake and Armando Iannucci.