The number of police CCTV cameras trained on Britain's roads has almost doubled over the last three years, giving police forces and the intelligence agencies access to up to 26 million images a day, the Guardian can reveal.

There are now more than 8,000 cameras in the network, with senior officers hopeful of extending it further because they regard it as a key tool that helps to cut crime and save lives.

However, the scale of the system is causing privacy campaigners grave concern and senior officers admit they need to do more to address fears about suspicion-less surveillance.

A national database that stores pictures taken by the automated numberplate recognition (ANPR) cameras currently has 17 billion images in its archive – thought to be the biggest of its kind anywhere in the world.

That number is likely to increase over the next five years with advances in technology and the roll-out of more fixed and portable cameras.

Although there is no specific police target, forces believe it should be possible to read and store between 50 million and 75 million "reads" a day by 2018.

Police also want to link the UK database – called the National ANPR Data Centre (NADC) – with similar systems run by other countries in Europe.

Julian Blazeby, who is the lead on ANPR for the Association of Chief Police Officers, said the system has become "one of the jewels of modern policing".But he conceded the cameras had concerned human rights groups and said the police needed to be more transparent about a system which has been shouded in secrecy.

"It is always a challenge for us, balancing the rights of individuals with preventing and detecting crime. However, we want to be as transparent as possible, and perhaps in the past we have not been as open as we could have been," he said.

"We want to reassure the public we are doing everything we can do prevent the misuse of the systems, with national standards and guidelines in place. ANPR is about helping the public to stay safe. I understand and respect people's concerns and I think we can address them."

But Nick Pickles, director of Big Brother Watch, raised concerns about the scale of the expansion of the system."ANPR is a classic example of a very intrusive technology being rolled out with zero public debate," he said.

The ANPR system has become a cornerstone of British policing over the past decade, but details of the number of cameras in operation, and the number of images on the database, are not publicly available.

The Home Office refused to give the figures to the Guardian or to detail how much police forces had spent on camera systems over the past three years.

However, the Guardian understands 46 forces in England and Wales have signed a memorandum committing themselves to share information to allow "the full exploitation of ANPR at national regional and local levels". Two images are taken of every vehicle – one focused on the numberplate, and a second of the whole car, which often includes the face of the driver. Details on the time of day and direction of travel are also kept.

The pictures can be kept for up to two years and cross-checked for "hits" against the Police National Computer and other "hotlist" databases, including the Driver and Vehicle Licensing Agency and the Motor Insurers' Bureau.

Speaking to the Guardian, Blazeby, an assistant chief constable in Staffordshire, insisted ANPR was vital for counter-terror and organised crime operations for the simple reason that "most criminals use the roads".He said: "There are myths about ANPR. It is actually a fairly basic technology and I am keen for police forces to maximise the use of it appropriately."

He admitted bulk databases such as ANPR and those held by insurance companies were becoming more important to the police. "There are numerous databases available to help in fighting crime. One of our priorities is to develop ways in which we can more effectively analyse this data. We also want a system that we can harmonise with Europe, so that ANPR is developed with our European police and law enforcement partners. It is not so much the technology that is changing, but police are embracing the benefits of the technology. We are committed to ensuring that we only use ANPR cameras where they are necessary, and we need to further develop ways in which we are able to analyse the data to prevent and detect crime."

Blazeby said there were no plans to upgrade ANPR cameras with facial recognition technology, though he left the door ajar. "I am never going to say never, but it is not in the pipeline at the moment and I do not see that changing in the foreseeable future."

Privacy campaigners said the expansion of ANPR was happening without proper scrutiny or debate, and raised concerns about which bodies have access to the vast amount of material on the NADC.

A police review of the system has also acknowledged serious flaws that are now being addressed.

Pickles, of Big Brother Watch, said: "The public hasn't been given even the most basic information about what information is being recorded, how long it is kept or who is able to access it. When you consider the system allows every journey to be tracked, down to the second, and for someone to search through the entire log of when your car was scanned and where, it is without doubt an extremely intrusive system. How many people would have said yes if they had been asked to install a device on their car so the police could monitor their journeys? Yet this is exactly what ANPR technology allows."

Pickles said ANPR was originally meant to record the details of cars that were already on a police watchlist. But forces had now begun recording every vehicle that passed one of the cameras.

"It is a total failure of law and oversight if this massive yet secretive expansion is legal."

The campaign group No CCTV said the police were becoming more and more like an intelligence agency, gathering details about people's lives that it did not need to have.

"The police talk about ANPR as if it is a magic bullet, and that they can't do without it," said a spokesman. "But the truth is they are storing details about millions of innocent people, and it is not at all clear who has access to this data. There has been a huge shift in British policing that has gone almost unnoticed. They have become intelligence gatherers like the spy agencies."

The first ANPR cameras were introduced in 1984 on the M1 to help identify stolen vehicles. In 2005 the government approved a scheme to set up 2,000 fixed cameras nationwide and a data centre.

Camera numbers have increased steadily since then and the Metropolitan police has equipped more than 100 police cars with mobile camera units. Known as ANPR interceptors, they were introduced two years ago to patrol central London, feeding information into a new bureau at Scotland Yard.

Police enthusiasm for the technology has raised concerns over privacy, and the Information Commissioner's Office slapped down Hertfordshire police last July for introducing a "ring of steel" around Royston, saying it could see no reason why the force thought it necessary to have cameras monitoring all traffic in and out of the rural town 24 hours a day.

"We hope that this enforcement notice sends a clear message to all police forces, that the use of ANPR cameras needs to be fully justified before they are installed," said Stephen Eckersley, the ICO's Head of Enforcement.

In 2010 a review criticised West Midlands police for spending £3m setting up 200 cameras in Muslim communities in Birmingham. The force apologised.

Last year the police acknowledged it had to work harder to justify the use of ANPR following three cases investigated by the Independent Police Complaints Commission.

One looked at the murder in October 2009 of 17-year-old Ashleigh Hall; her killer, a known sex-offender, had been picked up six times by ANPR on the day of her disappearance, but the leads were not followed.

The IPCC warned that the ANPR system was being overused for minor issues and higher priority matters were being missed.

The suicide of Sean Toombs in October 2010 also caused concern. Toombs was arrested by Lincolnshire police on suspicion of rape after ANPR registered a "hit" on his car.

Officers tried to stop him again the following day because his details had not been immediately removed from the ANPR list. Toombs failed to stop and died in his car after setting fire to a petrol can.

An ACPO review of ANPR conducted last year concluded: "Even completely innocent individuals can suffer the most serious consequences if ANPR is not accurate or not used or managed in an appropriate and managed way."In an effort to rebuild confidence in the technology, police have committed to abide by 14 "golden rules" to govern the use of ANPR.

Last month Acpo also launched a website to explain the benefits of the system.

A new Home Office code of practice states the cameras should only be used "in pursuit of a legitimate aim" and when it "meets a pressing need".

"ANPR is a vital tool and we support its use where it is legitimate and proportionate," a spokesman said. "We have created a new, independent post of surveillance camera commissioner to help oversee this. These cameras may only be located where there is an identified need to detect, deter, and disrupt criminality. Any new ANPR camera deployment will also require a privacy impact assessment."