Journalists at the Guardian and Observer will stage two 48-hour strikes over the planned sale of the Sunday newspaper.

The National Union of Journalists (NUJ) said its members will walk out on December 4-5 and again on December 12-13 following a “resounding” ballot result in favour of industrial action over the planned sale of The Observer to Tortoise Media.

The NUJ said it had repeatedly urged the Guardian Media Group (GMG) to halt talks with Tortoise Media and to engage with the union on the future of the title.

GMG has acted in poor faith, revealing hours into yesterday's negotiations that recommendation of the sale had already been agreed despite previous assurances to the contrary

Michelle Stanistreet, NUJ general secretary, said: “Journalists have delivered a clear message to the Guardian Media Group and Scott Trust over their resolve in this dispute.

“Industrial action is always a last resort, but deeply disappointed members have been left with no choice than to take action in their defence of the Observer’s heritage and public interest journalism.

“GMG has acted in poor faith, revealing hours into yesterday’s negotiations that recommendation of the sale had already been agreed despite previous assurances to the contrary.

“Members at the Guardian and Observer have the full support of the NUJ as they exercise their right to strike in a move that demonstrates the unity and strength of feeling among journalists.

“Industrial action can be avoided by the company pausing the sale to Tortoise Media and listening to the concerns of journalists and readers.”

A Guardian spokesperson said: “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.”