Journalists at two national newspapers are launching a 48-hour strike on Wednesday in protest at the planned sale of one of the titles.
Members of the National Union of Journalists (NUJ) at the Guardian and Observer are walking out for the first time in more than 50 years.
They are protesting at the planned sale of the Observer to Tortoise Media.
The journalists are mounting a picket line outside the Guardian and Observer headquarters in London.
The union said the strike coincides with the Observer - the world's oldest Sunday newspaper - marking 233 years since it first began publication.
NUJ general secretary-elect Laura Davison said: "Guardian and Observer members have the full backing of the NUJ as they undertake this significant industrial action - the first for over 50 years.
"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. The Observer holds a unique and important place in public life and our members care about the next chapter in its history.
"The union is calling for a pause in the exclusive talks to provide more time to consider alternatives and ensure decisions are in the best interests of both titles."
Ahead of the first strike day, wildlife TV presenter and conservationist Chris Packham issued a video of support on his social media and posted: "I don't think we should put a price on the truth. It's hard to find these days. But we do find it in the Observer and the Guardian.
"There are moves to sell the Observer which might imperil its future. Not good, that's why I stand in solidarity with its striking journalists."
A Guardian spokesperson said when the strike was first announced: "We recognise the strength of feeling about the proposed sale of the Observer and appreciate that NUJ members wish to make their views heard.
"While we respect the right to strike, we do not believe a strike is the best course of action in this case and our talks with the NUJ continue.
"Our priority is to serve our readers and support our staff, so that the Guardian and the Observer can continue to promote liberal journalism and thrive in a challenging media environment."
