More than a fifth of the public’s reports to terror police help foil plots and catch terrorists, but the number of tips has more than halved in a year.

The head of national counterterror policing warned that public complacency would be a “worst-case scenario” for security services, who have thwarted 18 attacks in under two years.

Metropolitan Police assistant commissioner Neil Basu said he was “incredibly grateful” that 2018 was spared the level of carnage of the previous year, when 36 victims were killed in atrocities in Manchester and London.

But with the number of ongoing terror investigations at a record of 700, Mr Basu said the flow of intelligence from members of the public was vital.

“I know some people are still reluctant to speak to us,” he acknowledged. “To them I say, reporting your concerns to us won’t ruin lives, but it might save them.”

In 2017, more than 31,000 reports were made to counterterror police, but last year the number fell to 13,000.

In both years, more than a fifth of tips were “very significant”, leading to the identification of a suspect or plot, or aiding prosecutions.

Mr Basu said part of the reason could be the national focus on Brexit, which “undoubtedly” took up much public attention last year, and the drop in attacks.

He said the terror threat had shifted to young, mentally ill and “malleable” people who may be affected by terrorist propaganda they view online.

London Bridge Terror Attack Show all 16 1 /16 London Bridge Terror Attack London Bridge Terror Attack Armed police on Borough High Street as police are dealing with a "major incident" at London Bridge PA London Bridge Terror Attack Armed Police talk to members of the public outside London Bridge Hospital as police are dealing with a "major incident" at London Bridge PA London Bridge Terror Attack Police Officers outside the Barrowboy and Banker Public House on Borough High Street as police are dealing with a "major incident" at London Bridge PA London Bridge Terror Attack Armed Police talk to members of the public outside London Bridge Hospital as police are dealing with a "major incident" at London Bridge PA London Bridge Terror Attack Armed police on Borough High Street as police deal with a 'major incident' at London Bridge PA London Bridge Terror Attack Emergency services near the scene of the incident Screengrab London Bridge Terror Attack People run down Borough High Street as police are dealing with a "major incident" at London Bridge Reuters London Bridge Terror Attack Emergency services arrive at the scene near Borough market at London Bridge Carl Court/Getty Images London Bridge Terror Attack Emergency personnel on London Bridge as police are dealing with a "major incident" at London Bridge PA London Bridge Terror Attack Police sniffer dogs on London Bridge as police are dealing with a "major incident" at London Bridge PA London Bridge Terror Attack A second helicopter lands on London Bridge as police are responding to three incidents in the capital, amid reports that a vehicle collided with pedestrians on London Bridge, Scotland Yard said. Officers are dealing with reports of stabbings in Borough Market, where armed officers attended and shots were fired. They are also at an incident in the Vauxhall area PA London Bridge Terror Attack Police attend to an incident on London Bridge in London REUTERS London Bridge Terror Attack Police attend to an incident on London Bridge in London, Britain Reuters London Bridge Terror Attack A police officer escorts members of the public to safety at London Bridge Getty Images London Bridge Terror Attack Police attend to an incident on London Bridge in London, Britain Reuters London Bridge Terror Attack Police attend to an incident near London Bridge in London, Britain Reuters

“They are taking that and then using very low-sophistication measures, things that everyone can get access to – a knife or a vehicle – and making an attack,” Mr Basu said.

“And those are the kind of things that worry us most, they are the hardest to see and they are the hardest to stop.

”And that’s why we need communities to stand up and report changes in behaviour that they are seeing within their communities which might actually help us stop these things before they happen.”

Police say 14 Islamist terror plots and four from far-right extremists have been foiled since the Westminster attack in March 2017. A total of 30 planned attacks were successfully disrupted in the four years before.

The Independent understands that the most recent plot security services classify as thwarted was an alleged Isis-inspired attack. The suspect is to stand trial later this year.

On Tuesday, the chief of MI5 updated cabinet on the current terror threat to the UK and work to deal with the risk from former “subjects of interest”, like the men who launched the London Bridge and Manchester attacks.

Andrew Parker told Theresa May there were “new ways of using data to detect threats” emanating from Isis, al-Qaeda and other groups.

Isis has been driven out of most of its ‘caliphate’ in Iraq and Syria but operates in several countries and continues to call for international attacks

The threat level to the UK from international terrorism remains at ”severe”, which means that an attack is considered highly likely.

Mr Basu also raised concerns about the impact of a potential no-deal Brexit, saying it would damage access to data and EU intelligence systems.

He said the UK would be put in a “very bad place” without continued exchanges or biometrics access, but that a team was working on contingency plans as counterterror liaison officers are “embedded” in embassies and agencies in Europe.

“What’s most concerning me about Brexit is its potential to divide communities and set communities against each other,” Mr Basu added, citing the rise in hate crime around the EU referendum and emergency of neo-Nazi group National Action.

“I’m concerned about the creeping rise of that threat, and if we aren’t clear that we are combating that threat then that’s my concern.”

Mr Basu was speaking at the launch of a cinema advert campaign to increase people’s awareness of suspicious activity and encourage them to report it to police.

The 60-second film, to be shown across the UK, shows a series of scenarios based on real-life plots, such as a man stockpiling hazardous material and another buying weapons.