A former Labour MP picked to carry out the review warned of “disruption or even violence carried out in the name of progressive causes”, as he started work.
John Woodcock, a former aide to Gordon Brown and now an independent peer, stressed that far-right groups remained a significantly bigger threat to British life.
But he said: “We must be vigilant against a similar blind spot in Britain to the prospect of progressive extremism.
“That is, unacceptable disruption or even violence carried out in the name of progressive causes to which the political establishment and large majority of the population have great sympathy, like climate change and racial injustice.”
Lord Walney, as he is now known, told The Daily Telegraph: “There have been a number of, at the moment isolated, examples of climate change activist groups, particularly Extinction Rebellion, overstepping the mark into antisocial behaviour.
“I think there’s been a recognition that, even among that movement, they have at times risked undermining their own cause.”
The peer added: “I’m coming at this with an open mind, but with an understanding that there is clearly a potential for groups to develop into increasingly problematic areas.”
But Gary McFarlane, a Black Lives Matter (BLM), activist said: “Tarring Extinction Rebellion (XR) and BLM with the same brush as far-right terrorists is an outrage.”
He described Lord Walney as “the hand-picked servant of a prime minister who described black people using vile racist language that thankfully most people left behind in the 1950s”.
“The truth is that both XR and BLM have wide support and encompass people of divergent political views united in common aims – and our rulers clearly find that a threat,” Mr McFarlane added.
Lord Walney, who quit Labour in July 2018 and endorsed the Conservatives at the 2019 general election, is Mr Johnson’s independent adviser on political violence.
He has said the role will “examine how fringe groups are using the pandemic to exploit fears and produce recommendations on the problem of activists on far right and far left hijacking legitimate causes with violence and law breaking”.
The peer also said people joining movements to achieve good causes are vulnerable to “a conscious or often unconscious bias in overlooking the strategies to get there”.
“I want to look at the way antidemocracy, anti-capitalist far left fringe groups in Britain like the Socialist Workers Party tend to have much more success hijacking important causes,” he told the Telegraph.
