Journalists at the Guardian and sister publication the Observer are holding a two-day strike in protest at the proposed sale of the Sunday newspaper to Tortoise Media.

The first strike at the Guardian in more than 50 years is taking place across Wednesday and Thursday (4 and 5 December).

Tortoise is run by former editor of the Times and ex-director of BBC News, James Harding.

A Guardian spokesperson said while they “recognise the strength of feeling” about the proposed sale and “appreciate that NUJ [National Union of Journalists] members wish to make their views heard”, they “do not believe a strike is the best course of action” at the present time.

“We have a plan in place to minimise the impact of strike action on staff, readers and subscribers and we will continue to publish online and produce the print edition as usual,” they continued.

UK readers may notice some differences to the Guardian’s daily website on Wednesday and Thursday, as a result of the strike, as well as in its print editions, the newspaper said.

NUJ members passed a motion last month stating that selling the Observer to Tortoise would be a “betrayal” of the commitment made by the Scott Trust, the Guardian Media Group (GMG)’s owners, to the world’s oldest Sunday newspaper.

If the sale goes ahead, Observer staff have been told they can take voluntary redundancy on enhanced terms or else transfer to Tortoise on the existing terms of their contracts.

In an email to staff, Anna Bateson, the managing director of GMG, confirmed “constructive talks [around the sale] will continue” in order “to protect Observer staff if this deal progresses”.

She added that freelance contracts would be extended until the end of September 2025 and then renegotiated.

Chair of the Scott Trust, Ole Jacob Sunde, reportedly told staff: “Throughout the process our goal has always been to do what is right for Guardian and Observer readers and staff so that both titles continue to promote liberal journalism and thrive long into the future.”

According to the Guardian, Sunde added that the Scott Trust would stay on as part-owner of the Observer in any such deal, and that the new owners would have to “embody the values of editorial independence, press freedom and liberal journalism”.

“I fully respect people’s rights to take industrial action,” Sunde continued.

NUJ general secretary-elect, Laura Davison, said the striking journalists “have the full backing” of the union as they “undertake this significant industrial action”.

“The massive vote to take this step shows journalists’ desire to publicly highlight to readers and those in charge their collective concerns about the future of the title,” she added.

She noted how the 233-year-old Sunday newspaper “holds a unique and important place in public life and our members care about the next chapter in its history”.

Harding has said his company’s proposed takeover offers the chance to invest in the title and extend its legacy.

It was revealed in September that Tortoise had approached the Guardian Media Group with an offer to invest around £25m over the next five years on the “editorial and commercial renewal” of the Observer.

A GMG spokesperson said: “We respect the right of NUJ members to strike. We continue to publish online and in print and are working to minimise the impact on staff and readers. Our priority is to make sure the Observer’s journalism continues to have a leading role in the liberal media landscape.”