One of the world’s most successful private equity chiefs is being lined up to help fund a takeover of The Observer, the world’s oldest Sunday newspaper.

Sky News has learnt that Patrick Healy, the chief executive of Hellman & Friedman (H&F), a US-based buyout firm, is among the Tortoise Media shareholders who are in talks to commit millions of pounds in a five-year investment plan for the title.

Talks about a deal between Guardian Media Group, The Observer’s owner for the last three decades, and Tortoise Media, a five-year-old start-up, have been ongoing for months.

They were disclosed publicly in September, and are believed to have weeks left to run before a formal agreement is reached.

The deal has sparked controversy among Guardian and Observer journalists, who argue that staff on the Sunday newspaper – which traces its roots back to 1791 – should be protected by the same safeguards as those provided to The Guardian by the Scott Trust.

Earlier this month, an open letter signed by leading figures from the arts and culture including Bill Nighy, Hugh Grant, Mary Beard and Ralph Fiennes labelled the prospective deal “disastrous”.

“While figures of £100m are being bid for other publications [a reference to the recent sale of The Spectator magazine], this poorly funded approach sets the value of the Observer at or near zero,” the letter said.

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“The proposal also envisages moving it from a resilient and well-funded newspaper publisher to a small, loss-making digital start-up whose funding for the takeover would in all likelihood come from private equity.”

Mr Healy is not the only Tortoise Media shareholder keen to participate in its new funding round, with David Thomson, the chairman of Thomson Reuters and Woodbridge, his family office, also thought to be interested.

Both Mr Healy and Mr Thomson are already identified as minority investors in Tortoise Media’s share register.

Mr Healy has a long pedigree as an investor in the media industry through H&F, having overseen the firm’s interest in companies such as Fairfax Media in Australia and Axel Springer, the German media group which last year explored an offer for The Daily Telegraph.

In 2015, he worked on a potential bid for the Financial Times before losing out to Nikkei of Japan.

There was no indication this weekend about the scale of Mr Healy’s potential investment in Tortoise Media to fund the deal, although a person close to the start-up said its discussions with investors had not yet concluded.

Tortoise Media has pledged to invest £25m in The Observer over a five-year period, although it is unclear whether it is trying to raise the entire sum before the transaction completes.

The company was founded five years ago by James Harding, a former BBC executive and editor of The Times, and Matthew Barzun, the ex-US ambassador to Britain.

It specialises in what it calls ‘slow news’, providing analysis and commentary on major events.

Mr Harding is understood to be planning to meet senior Observer staff next week amid the threat of strike action against the deal with Tortoise Media.

In a statement issued last month after Sky News revealed the discussions, Anna Bateson, GMG’s chief executive, described the deal as “an exciting strategic opportunity for the Guardian Media Group”.

“It provides a chance to build the Observer’s future position with a significant investment and allow the Guardian to focus on its growth strategy to be more global, more digital and more reader-funded.”

Katharine Viner, The Guardian’s editor-in-chief, said the development had “the potential to be a very positive thing for both the Observer and the Guardian”.

“My number one priority is a future in which both titles continue to thrive and deliver high-quality journalism to our readers,” she said.

“It is extremely important to me that the Observer, with its excellent journalistic reputation, loyal readership and heritage as the world’s oldest Sunday newspaper, is in good hands.”

Mr Harding has pledged to retain a print presence for The Observer, which was founded in 1791 by WS Bourne on the premise, according to an official history of the title, that “the establishment of a Sunday newspaper would obtain him a rapid fortune”.

Tortoise Media, which remains lossmaking, also counts Lansdowne Partners, a prominent Mayfair hedge fund, and LocalGlobe, a leading venture capital firm, among its investor.

“We think the Observer is one of the greatest names in news,” Mr Harding said last month.

“We will honour the values and standards set under the Guardian’s great stewardship and uphold the Observer’s uncompromising commitment to editorial independence, evidence-based reporting and journalistic integrity.

“George Orwell described the Observer as ‘the enemy of nonsense’; we’re excited to show readers, old and new, that it still is.”

This weekend, Tortoise Media and GMG declined to comment, while Mr Healy also declined to comment through a spokesman and Mr Thomson could not be reached for comment.