The UK is to start issuing official threat-level warnings for far-right terrorism amid rising concerns about white supremacist murder attempts, the Guardian has learned.

The threat levels will be issued following assessments by the Joint Terrorism Analysis Centre (JTAC), an elite Whitehall unit that already produces similar warnings for Islamist and Ireland-related terror.

Friday’s attacks in Christchurch, New Zealand, when 50 Muslims were murdered as they prayed, allegedly by a white supremacist gunman, have triggered fresh concerns about whether the threat from the extreme right is being taken seriously enough.

Combating far-right violence was once the responsibility of the police but top-level plots and suspects are now being tackled by Britain’s domestic intelligence agency, MI5. Public order issues and hate crime will remain with the police, and JTAC’s formal assessment of the far-right threat is expected to start this year.

There have been 18 terrorist plots thwarted in the UK since March 2017, 14 of which were inspired by Islamist extremism, assessed as much the greater threat, and four emanated from the far right.

However, counter-terrorism officials believe white supremacist terrorism is an increasing threat. The stabbing of a teenager in Stanwell on Saturday is being treated as an alleged far-right terror attack and a 50-year-old suspect remains in custody on suspicion of attempted murder. Other incidents have been reported since the Christchurch attack, including online gloating.

On Tuesday, Theresa May’s official spokesman said: “The incident which took place in Surrey on Saturday evening fitted a pattern of concern that the threat from the far right is on the rise.

“The prime minister said there could be no place for vile extremist views in society and people must be able to practise their faith without fear.

“She said that intensive work was taking place across government and by the police and security services to combat the threat.”

In 2017/18 of the 394 individuals who received support from the Channel program to counter potential violent extremism, 45% were referred for concerns related to Islamist extremism and 44% for concerns related to rightwing extremism.

Meanwhile, an Imam who attended an inclusion event with the home secretary, Sajid Javid, on Monday revealed he had been abused twice on his way home.

Mohammed Mahmoud said he was spat at and verbally abused after leaving the event at Regent’s Park mosque in central London. He gained widespread praise after saving Darren Osborne from an angry crowd after he staged a terror attack in June 2017 by mowing down worshippers leaving prayers at Finsbury Park mosque, leaving one man dead. Osborne was cited by the Christchurch attacker.

Mahmoud, who denounced the far right after the New Zealand atrocity, told the Evening Standard: “A middle-aged white male on the bus told me I was “despicable” and a “shithole” … When I asked why – he said it was because I was wearing a dress.”

In a second incident, he was spat at by a cyclist. Mahmoud said: “It was so sad to see such reckless hate and Islamophobia after the event and after all we have been through this week.”

The Home Office has announced more money for mosque security. It will rise to £1.6m, previously £2.4m had been promised over three years. Another £5 m will fund security training over three years.

Javid said: “I know many Muslim communities are feeling vulnerable and anxious. But they should seek comfort from knowing we are doing everything to tackle hate and extremism.

“That’s why we are doubling next year’s places of worship fund – providing physical protection as well as peace of mind.”

The Muslim Council of Britain welcomed the increase but said more needed to be done. “The increase in the Places of Worship fund, the simplified bidding process and the investment in non-physical security support, are all welcome first steps, in particular given the heightened concerns following the terrorist incidents targeting Muslims in New Zealand and at a smaller level, here in the UK.

“However, British Muslim communities may still question why the funding is not proportionate to the risks they face, given the Home Office’s own figures last year showing that over half of religious hate crime is targeted at Muslims.”