By Paul Sandle

LONDON (Reuters) -Britain’s Guardian said on Friday it had agreed to sell The Observer to startup Tortoise Media, which sought to overcome staff opposition by pledging to revive the world’s oldest Sunday newspaper as a strong voice in liberal journalism.

Tortoise, founded five years ago by ex-BBC News boss James Harding to focus on “slower, wiser news”, said it had raised 25 million pounds ($32 million) to invest in the newspaper.

The centre-left Observer, first published in 1791, is one of Britain’s best-known newspapers and once carried the journalism of 1984 author George Orwell.

It was acquired by Guardian Media Group in 1993, when it became a sister title to the Guardian daily. The Observer does not have a distinct online identity, with all its content published on the Guardian’s popular site.

Journalists at the Guardian and Observer were opposed to the sale and staged a 48-hour strike this week in protest.

The National Union of Journalists, which organised the strike, said it was concerned about “the sustainability of the Observer’s journalism if it is sold to the non-profitable startup Tortoise”.

Under the deal, GMG’s owner, the 1.3 billion pound Scott Trust, will take a minority stake in Tortoise.

Katharine Viner, editor-in-chief of Guardian News & Media, said she recognised how unsettling the sale was for Observer staff, but she was confident that the deal was the best possible way forward for its journalists and readers.

“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,” she said in a statement.

Tortoise said it would give the title its own digital identity, with a focus on publishing less breaking news and more narrative investigations, eyewitness reporting and data journalism.

“We will put The Observer’s online content behind a paywall, following The Atlantic’s highly successful revival by adapting to today’s media environment,” it said.

“We believe in its future, both in digital and as a multi-section newspaper published each and every Sunday.”

A representative from The Scott Trust will join its company and editorial boards, Tortoise said.

Company filings show that Tortoise counts Woodbridge Investments Corporation, an investment vehicle for the Thomson family who are majority owners of Thomson Reuters, among its backers.

($1 = 0.7837 pounds)

(Reporting by Paul Sandle, Muvija M and Sachin Ravikumar; Editing by William James, Kate Holton and Alexander Smith)