Journalists and editors at the French newspaper Le Monde have signed an open letter demanding their owners guarantee the paper’s editorial independence by giving them approval over who holds controlling shares in the company.

The letter, published on Tuesday and signed by two of Le Monde’s directors as well as 450 journalists, gave two major investors in the paper a deadline of a week to agree.

Staff say without an agreement, Le Monde is facing a threat to its editorial freedom for the first time in its history.

In 2010 the paper was acquired by three private investors: the telecoms tycoon Xavier Niel, the former banker Matthieu Pigasse and the industrialist and Yves Saint Laurent co-founder Pierre Bergé, who died in October 2017. Staff voted to approve the sale.

A year ago Le Monde’s editorial director, Jérôme Fenoglio, says staff were shocked to discover Pigasse had sold 49% of his shares to the Czech energy industrialist Daniel Křetínský. Le Monde’s Independency Group, a minority shareholder made up of journalists, other staff, readers and founders, was not informed of the sale.

Fearing a new powerful owner was about to be forced on them, the group asked Niel, Pigasse and Křetínský to sign an “approval agreement” allowing it to endorse or reject any new shareholder with controlling rights.

Niel has since signed the agreement, but Fenoglio says Pigasse and Křetínský have not. The letter is intended to persuade them to do so and inform readers of the potential threat to editorial independence.

“We have a sense of independence that we are prepared to defend,” Fenoglio told the Guardian. “This [letter] is not intended to be bellicose but to explain what we are and why it is important that we should be able to approve any new potentially controlling shareholder. It’s not an act of war; it’s a request that they sign.”

The letter said: “The right of approval for new controlling shareholders is an essential document to complete and reinforce the fragile mechanism protecting our work.”

It continued: “It’s about giving a legal form to the spirit of the relationship established in 2010 – when the newspaper was purchased – with Mr Bergé, Niel and Pigasse, whom we chose by vote. Since then, there has been no exception to this rule of separation between capital and journalists.”

Le Monde’s director of editorial staff, Luc Bronner, added the letter was unanimously backed by staff.

“Up until now the major shareholders have let us write whatever we want. Even when we have written about them there has been no problem. They have understood this is what makes Le Monde what it is,” he said.

“Now we have been negotiating for a year and we would like Matthieu Pigasse and Daniel Křetínský to sign this agreement. Otherwise it is damaging to the newspaper.”