“The groups are using disruption and their consistent manipulation of the truth to suit their own personal and political agendas,” it added in a statement.
Police said they had so far made 63 arrests at the two locations.
Newsprinters, which runs the plants, said printing had been transferred to other sites and apologised to customers “due to late deliveries”.
The Times also apologised to readers unable to buy the newspaper and said on Twitter it was “working to get newspapers delivered to retailers as soon as possible”.
The blockade prompted an immediate backlash from across the British political establishment.
“This morning people across the country will be prevented from reading their newspaper because of the actions of Extinction Rebellion,” Interior Minister Priti Patel said on Twitter.
“This attack on our free press, society and democracy is completely unacceptable.”
The main opposition Labour Party’s international trade spokesperson Emily Thornberry called the disruption “wrong”, noting elderly people could miss out on newspaper deliveries.
Even Prime Minister Boris Johnson’s fiance Carrie Symonds, a former head of communications for the ruling Conservative party before becoming an environmental campaigner, waded into the controversy.
“I care about climate change and biodiversity a massive amount but preventing a free press to spread this message further is just wrong,” she tweeted.
“Not to mention all those small businesses that rely on being able to sell newspapers.”
A spokeswoman for Newsprinters said the action had impacted many workers within the industry.
“Overnight print workers, delivery drivers, wholesale workers and retail newsagents have faced delays and financial penalty,” she added, noting it was “a matter for the police” and government.
‘Dear Mr. Murdoch’
XR said on Twitter it was sorry for disruption caused to small stores selling newspapers but was unrepentant about its targeting of the media conglomerates.
“Dear Mr. Murdoch, we are absolutely not sorry for continuing to disrupt your agenda this morning,” it added.
The group, which formed in Britain in 2018 before becoming a global protest movement, kicked off 10 days of renewed demonstrations across the country on Tuesday.
In its third major wave of UK protests in a year, it has targeted Westminster and several other sites so far this week, and is vowing to continue with further demonstrations in the coming days.
Police have been taking a tougher approach towards the group during this round of protests, imposing restrictions at sites and making hundreds of arrests.
Last year, more than 1,700 people were arrested during its 10-day “Autumn Uprising”, which saw major disruption across the UK and large parts of central London blocked off.