“The right-wing terrorist threat is more significant and more challenging than perhaps public debate gives it credit for.”

The words are those of Mark Rowley, Assistant Commissioner of London’s Metropolitan Police as he revealed that detectives had foiled four plots by right-wing extremists in the last year.

In his valedictory speech, he described the organized nature of far-right movements as something new, warning that British groups were seeking links with international extremists.

“It’s a significant part of the terrorist threat. Right-wing terrorism wasn’t previously organised here,” Rowley said.

"There are many Western countries that have extreme right-wing challenges and in quite a number of those the groups we are worried about are making connections with them and networking," he said. He declined to give further details as investigations are ongoing, he added.

Britain has been a prime target for Islamist militants since the 9/11 attacks on the United States in 2001. Four years later more than 50 people were killed and hundreds injured when home-grown terrorists launched a series of bombings in the name of political Islam.

The same purported motive was again seen last year, which saw five deadly attacks in Britain. One though was carried out by a man who – radicalised on far-right material – rammed a hired van into Muslim worshippers outside a mosque in north London, killing a grandfather. The previous year Labour MP Jo Cox was murdered by a far-right extremist.

Since the first of the attacks in March 2017, police have foiled 10 Islamist conspiracies and four far-right plots, Mark Rowley says. Some 600 terrorism investigations are ongoing involving more than 3,000 suspects.

The retiring police chief says far-right and Islamist groups feed off each other, pursuing the same extremist agenda and often using the internet to fan the flames.

Earlier this month a neo-Nazi was convicted of plotting a terrorist attack at a gay pride event in Cumbria, northwest England. He was arrested on the way to the pub where he was planning the attack following a tip-off from a member of a far-right Facebook group; Nazi material and weapons including a machete were found at his home.

What stands out as new is the growth of organised extreme-right networks, police say. Since the murder of Jo Cox, National Action and two other spin-off groups have become the first far-right organisations to be outlawed since the 1940s.

“For the best part of 18 months in the UK we have a homegrown, white supremacist, neo-Nazi terrorist organisation that is pursuing all the ambitions of any other terrorist organisation committed to violence,” the assistant commissioner says.

“That should be a matter of great concern for all of us.”