The nuclear industry funds the special armed police force which guards its installations across the UK, and secret documents, seen by the Guardian, show the 750-strong force is authorised to carry out covert intelligence operations against anti-nuclear protesters, one of its main targets.

The nuclear industry will pay £57m this year to finance the Civil Nuclear Constabulary (CNC). The funding comes from the companies which run 17 nuclear plants, including Dounreay in Caithness, Sellafield in Cumbria and Dungeness in Kent.

Around a third is paid by the private consortium managing Sellafield, which is largely owned by American and French firms. Nearly a fifth of the funding is provided by British Energy, the privatised company owned by French firm EDF.

Private correspondence shows that in June, the EDF's head of security complained that the force had overspent its budget "without timely and satisfactory explanations to us". The industry acknowledges it is in regular contact with the CNC and the security services.

Most of the nuclear force's officers are armed with high-powered guns and Tasers. The CNC has spent £1.4m on weapons and ammunition in the past three years.

They patrol outside nuclear plants, with their jurisdiction stretching to three miles beyond the perimeter of the installations. They have the same powers as any other British police officer and can, for instance, arrest and stop and search people.

The body that regulates the CNC is also funded by the nuclear industry. Four of the eight members of the Civil Nuclear Police Authority are nominated by the nuclear industry as its representatives. Those four are employed in the industry. The others – mainly former police officers – are deemed to be independent.

The force is expected to expand as the government presses ahead with plans for a new generation of nuclear plants, which are likely to attract protests.

Ben Ayliffe, head of Greenpeace's anti-nuclear campaign, said: "There are very obvious worries about an armed police force that is accountable to an industry desperate to build nuclear reactors in the UK. This industry will probably be very keen for their police force to use all the powers available to them to prevent peaceful protests against nuclear power."

John Sampson, the CNC's deputy chief constable, said the force was by law operationally independent from the industry and safeguarded by its regulatory police authority. Its surveillance operations were only conducted if they had legal approval and were proportionate to the crime under investigation, he added.

Sampson said it would be "ill-advised" of any nuclear company to put pressure on the force and surveillance was not conducted at the instigation of the companies. The government has provided a small amount of capital funding but does not pay any of the running costs.

The job of the force is to protect civil nuclear plants and guard radioactive nuclear material when it is being transported by land, rail or sea to ensure it is not stolen or sabotaged. The industry also receives advice from the security services on how to protect itself from attack.

The force is authorised to send informers to infiltrate organisations and to conduct undercover surveillance under the Regulation of Investigatory Powers Act 2000 (RIPA). It is also permitted to obtain communications data such as phone numbers and email addresses.

Reports by Sir Christopher Rose, the watchdog responsible for inspecting the use of these surveillance powers, have been obtained by the Guardian under freedom of information legislation.

Rose, the chief surveillance commissioner, noted last year: "The strategic aims of the constabulary remain on the threat from terrorism and public disquiet over nuclear matters, including demonstrations/protests and criminal offences towards nuclear movements/installations."

The force keeps secret the extent of its clandestine surveillance operations on protesters and others. It has been collecting more intelligence in recent years.

Sampson said its surveillance was "relatively modest" and mainly concerned with stopping terrorism.

In July, Rose said the CNC's "approach to covert activity is conspicuously professional". He found that the system for storing the intelligence gained from informers was "working well".

He says he has been told during inspections that "senior officers regard covert surveillance as a long-term requirement".

Since 2007, the CNC has also been headed by an ex-intelligence official, rather than a police officer. Richard Thompson is reported to have been a senior officer in MI6. Rose noted Thompson "has extensive experience in the intelligence world, but has no previous police background".