They are secretive, seemingly unaccountable and very rich. The network of agencies set up to run Britain’s speed cameras are raking in more than £100m a year from penalties. Many of them are finding the money from prosecuted motorists is rolling in far faster than it can be spent.

Critics are now beginning to question where such vast sums of money are going, and pointing to an ever growing bureaucratic gravy train built on speed cameras. The result is that motorists are footing annual bills of up to £100,000 for running each camera — about the cost of three uniformed police officers.

The business of fining motorists and ploughing back the money into installing more cameras is so successful that the number of agencies running them has grown from just eight to 42 in only three years. The agencies call themselves “safety camera partnerships” (use of the word “speed” is deliberately avoided). The name conveys the impression of public-minded co-operatives working with the sole aim of cutting the numbers of road casualties.

In reality, the partnerships may be non-profit making but they are quangos using an apparently inexhaustible source of cash from motorists’ fines to finance their expanding empires. Most partnerships are led by a board of representatives from the local council, police and magistrates’ courts, and in some cases private engineering firms act as project managers.

The partnership buys the services of police and council workers. Their staff continue to work at police stations or council offices, but they are on the payroll of the partnership.

The consequence of this complicated system appears to be spiralling costs and large amounts of money regularly disappearing into overtime payments and expenses. Moreover, scrutinising the partnerships’ use of public money is difficult because they do not make their accounts public.

However, The Sunday Times has obtained accounts for the Essex partnership which, along with London, Thames Valley and Lancashire, is one of the country’s biggest. It maintains speed cameras in an area covering around 5,000 miles of roads and a population of 1.6m.

The Essex partnership is managed by Atkins Highways and Transportation, a civil engineering consultancy. Last year the partnership was awarded £6.2m by the Department for Transport — its share of an estimated £142m collected nationally in fines. About three-quarters of the £142m was handed to the partnerships to cover costs and pay for installation of more cameras, but because they could not find enough to spend the money on, the remaining quarter went to the Treasury.

From its Chelmsford headquarters, the Essex camera partnership “buys” services from the local authority, the police and the magistrates’ courts. It employs the equivalent of 53 people and its costs are burgeoning. Last year it spent more than £5.15m — an average of £100,000 for each of the 51 roadside cameras it maintains.

Intriguingly, the accounts show that the cost of processing film from Essex’s speed cameras — a sum most people might reasonably think would account for the lion’s share of costs — amounted to just £18,693 in the period 2002-3.

In fact, the partnership spent most of its money on administration. Essex magistrates’ service was paid £222,000 to send out letters and summonses, while Essex county council was paid some £2.24m, including £16,730 for advertising and publicity and £70,392 for salaries. Atkins’s cut was £237,126 in consultancy fees.

But half of the camera partnership’s total went to Essex police, which spent £2.44m on administration, overtime and expenses. Reimbursement claims obtained by The Sunday Times reveal £50,198 on stationery (not including £24,150 on photocopy paper), £43,988 in travel expenses, £38,888 in subsistence — including snacks and meals out — £32,969 in police overtime, £8,279 for mobile phones and pagers and, bizarrely, £4,667 for interior decoration. There is even a claim for £64 for a pair of spectacles.

Critics say that the money from fines is being used to offset other police and council costs. Even after all these bills were paid, some £1m “surplus” was left, which according to Atkins was carried over to the new financial year.