THE Daily Mirror's political editor has called Boris Johnson out after he spoke out on "vital" free press.

The Prime Minister was referring to an environmental protest at printing presses for some of the UK's major newspapers, calling them "completely unacceptable".

More than 100 Extinction Rebellion (XR) protesters used vehicles and bamboo lock-ons to block roads outside the Newsprinters printing works at Broxbourne, Hertfordshire, and Knowsley, near Liverpool on Friday night.

The Newsprinters presses publish the Rupert Murdoch-owned News Corp's titles including The Sun, The Times, The Sun On Sunday and The Sunday Times, as well as The Daily Telegraph and Sunday Telegraph, the Daily Mail and Mail On Sunday, and the London Evening Standard.

By Saturday morning, police said some 72 people had been arrested during the action, which delayed deliveries of papers to newsagents on Saturday morning.

READ MORE: Extinction Rebellion protesters demonstrate at Scottish Sun printers

The demonstrations ended shortly after 11am and the lock-ons were dismantled.

XR protesters also held a smaller demonstration near Motherwell aimed at disrupting the distribution of Saturday's Scottish Sun newspaper.

Johnson took to Twitter to criticise the events, stating: "A free press is vital in holding the government and other powerful institutions to account on issues critical for the future of our country, including the fight against climate change.

"It is completely unacceptable to seek to limit the public's access to news in this way."

But the irony in his statement did not go amiss, with political editor Pippa Crerar pointing out the PM's hypocritical take.

She replied: "I agree with this completely. Which is why it surprised me you banned the Mirror from the Tory election bus, selected media from civil service briefings and refused to put ministers up on Channel 4, GMTV etc.

"Get your own house in order."

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In February this year, a dispute arose after Johnson's director of communications, Lee Cain, banned selected journalists from attending a Downing Street briefing.

Cain tried to exclude reporters from publications including the Mirror, the i, the Independent and HuffPost.

Johnson also caused a stir in the lead up to December's General Election after avoiding multiple media appearances, including an interview with Andrew Neil and a planned televised leaders debate on Channel 4.

Many Twitter users applauded Crerar's response, with one saying: "I read his tweet and thought he'd been hacked."