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Policing in England and Wales could be at "significant" risk if resources are diverted to fight terrorism, the UK's top counter-terrorism officer has said.

Counter-terrorism policing has been placed on an "emergency footing" after recent attacks, the BBC understands.

Assistant Commissioner Mark Rowley has asked ministers to reassure officers that funding will not be diverted from mainstream policing as a result.

The Home Office said counter-terrorism funding would increase by 30% by 2022.

'Difficult choices'

The BBC understands that Mr Rowley has written to Home Secretary Amber Rudd, warning that the counter-terrorism policing network was not able to operate at "full strength".

"The demand for increasing numbers of detectives in areas such as child abuse has prevented this," he wrote.

He suggested that prioritising counter-terrorism work would involve "difficult choices" about where to put resources.

"It will inevitably push risk to other areas of policing, potentially with significant impact," he said.

The letter, which was sent a week ago, before the Finsbury Park attack, included a plea to avoid "uncertainty over funding" so chief constables did not "shy away" from important operational changes.

The Met has said that it is discussing funding issues with the government.

'Four-fold increase'

There are no formal talks, just regular dialogue over a range of issues, including funding.

The three-month emergency plan, known as Operation Roset, was put in place after the first three attacks this year - at Westminster, Manchester Arena and London Bridge.

The operation is designed to intensify counter-terrorism activity - in particular the capacity of police to investigate plots - following a four-fold increase in the number of leads they were following up with security service MI5.

But with more than 700 officers and staff dealing with inquiries relating to the London and Manchester attacks, police must find resources for counter-terrorism work from elsewhere.

It is understood some staff will be temporarily removed from the ongoing public inquiry into undercover policing.

Image copyright Reuters Image caption This year's terror attacks, including the bomb at Manchester Arena, have put police on an emergency footing

Various war crimes investigations will also be suspended and officers will be transferred from regional organised crime units.

Staff will be diverted from other counter-terrorism functions, which BBC News has decided not to specify to avoid compromising national security.

In the letter, Mr Rowley said police and crime commissioners, though supportive of Operation Roset, were concerned about the implications for other policing areas.

"They want reassurance that financial resources will not be diverted from mainstream policing," he wrote.

'Magic police tree'

The UK has had an "exceptional" style of policing at a local level for almost 200 years, but a former head of the national counter terrorism office said this had changed.

Speaking on Radio 4's Today, Chris Phillips said: "I don't remember anyone telling us that that style of policing was going to disappear.

"Well I can tell you in reality it has pretty much already disappeared and I don't think the public are really happy with that… they won't be happy with that if they knew."

Mr Phillips told the programme every day hundreds of officers were being deployed to deal with different inquiries and policing was "absolutely at breaking point".

"Those officers have not just come from some magic police tree with officers available for deployment, they've come from the boroughs, they're coming from local policing."

'Extremely challenging'

The National Police Chiefs' Council (NPCC) confirmed that plans were being implemented to deal with the heightened terror risk.

It said the service was facing an "extremely challenging period", with some officers having to work overtime to fill gaps.

A spokeswoman said: "We are facing an unprecedented terror threat and it is no surprise that our resources are currently tested against what is now four terrorist attacks and five thwarted plots in very short succession."

Image copyright PA Image caption Mr Rowley suggested measures to bolster security, but said they were in themselves "unsustainable"

Mr Rowley's letter, which was copied to leading figures in security and law enforcement, also contains a list of measures to bolster physical security, make more use of de-radicalisation programmes and improve preparedness for an attack.

But he said these measures were "no more than an immediate patch".

He added: "In themselves they will not be enough. They are also unsustainable."

Second letter

The BBC understands that a second letter with more detailed points was sent to Ms Rudd - this time signed by Mr Rowley, Cressida Dick, the Met Police Commissioner, Lynne Owens, head of the National Crime Agency, and Sara Thornton, who leads the NPCC.

Speaking to the London Assembly on Wednesday, Ms Dick confirmed that resources were being shifted to counter-terrorism work from other policing areas because of the workload created by the terror attacks in London and Manchester.

The commissioner said there was an "impact" on other investigations, with some having to "pause" while others have had to "slow down", echoing concerns raised in the letter to the home secretary,

The commissioner said the attacks at Westminster, Manchester Arena, London Bridge and Finsbury Park had left the counter-terrorism police network "stretched".

She said they involved a lot of "backward-looking resource" which had taken away resources from pro-active inquiries.

Asked about the way police had handled calls to the anti-terrorist hotline, amid claims that concerns about one of the London Bridge attackers had not been acted on, she said her information was that the calls had been "logged, assessed and dealt with properly".

She said there would be reviews of the incidents and suggested that there might have to be greater regulation of van hire.

Ms Dick said the terror attacks had taken place against a background of increasing demand on policing in London, with emergency calls on the rise, a surge in knife crime and, over the past two months, a "higher than average" number of murders.

'Stretched blue line'

Jack Dromey MP, Labour's shadow police minister for three years, said: "Britain is facing the most serious threat of terrorism in a generation and revelations from Britain's top police officers show that the thin blue line is being stretched to breaking point."

He added that police did not have the "numbers and resources necessary to keep the public safe".

The Home Office said it had agreed in 2015 that overall police funding would be protected in real terms - and that cross-government spending on counter-terrorism would rise from £11.7bn to £15.1bn.

A spokesperson said: "Keeping families, communities and our country safe is this government's priority.

"After the recent horrific attacks, the government and police are in complete agreement that we must review our counter-terrorism strategy to tackle the changing threat."