Councils are making millions of pounds from a private company accused of using aggressive and intimidating tactics to issue fines for minor offences such as littering and dog-fouling, an investigation has found.

In interviews with the Guardian, whistleblowers said staff working for Kingdom Services Group – one of the fastest growing private security and services firms in the country – had targeted elderly people for littering because they knew they would be more likely to pay the fines.

And they alleged that, with the company’s encouragement, some officers had deliberately targeted vulnerable people in deprived areas. One former officer said: “I believe this company is profiting from poverty.”

Images on Kingdom’s website show staff conducting legitimate operations on behalf of councils. The company says that its officers undertake a comprehensive training programme and “operate under some of the tightest legal guidelines”.

But whistleblowers say this does not tell the full story. Through interviews, leaked internal messages and freedom of information requests, a Guardian investigation found that Kingdom:

• Is alleged to have encouraged staff to compete to issue as many fines as possible through a “100 club” league table in at least one area.

• Is accused of deploying a training team that encouraged staff to conceal logos on their uniform and hide in bushes or behind cars to improve their chances of catching members of the public.

• Is alleged to have followed people or falsely accused them in order to impose fines.

• Generated £1.4m in fines over just eight months in one council area alone.

A woman who successfully challenged a fine issued by Kingdom in the north-west told the Guardian she believed officers were issuing fines to people who were “easy targets”. The 57-year-old said two officers had approached her car in a supermarket carpark and accused her of dropping a cigarette. The woman said the company had only dropped the complaint after she obtained CCTV from the supermarket proving she had not thrown away her cigarette.

While some councils have dropped the company, at least 14 still use it to collect fines. Kingdom denied any misconduct and said it maintained “high standards”.

There is growing concern over how the company fulfils its contracts. Photograph: YouTube

But critics said that the claims showed that council outsourcing of the fining process was out of control. The human rights charity Liberty said: “We have always said these powers are ripe for misuse. The powers should be scrapped from the statute book.”

Kingdom is tasked by councils with dealing with a range of minor offences. Data obtained by the Guardian shows that the company has issued thousands of fines for cases including littering, dogs being off leads, dog-fouling and breaches of public space protection orders. More than 10,000 fines have been issued in Bristol alone by Kingdom in just under a year from November 2017, worth nearly £900,000.

The company holds contracts with local authorities in England and Wales that routinely include a revenue-sharing agreement. Last year, a contract between the company and Liverpool city council generated a total of £1.4m in just eight months. And Kingdom’s website says that more broadly contracts have resulted in £3.3m for local authorities during the last 12 months.

That success has coincided with a period of growth for Kingdom. Turnover at the company, which was known as Kingdom Security Limited until 2016, has surged from £27.7m in the 2012-13 financial year to £103m in 2017-18.

But there is growing concern over how the company fulfils its contracts. Internal messages provided to the Guardian by a former Kingdom officer based in the north-west of England appear to show area management competing to issue as many fines as possible, drawing up a “100 club” league table for officers who issue at least 100 fines a month, equivalent to £10,000.

“It was an internal leaderboard,” the former employee said, speaking anonymously. “It was all about targets and bonuses … In my training, if you saw someone smoking you could wait to see if they dropped their cigarette butts.”

Another former Kingdom officer, who worked in Wales before leaving the company in 2017, alleged there was a “culture of fear” and officers had to issue four tickets a day for the contract to be financially viable. When they had problems doing so in winter due to less footfall a training team was brought in.

“The training team came in and caused mayhem … You are supposed to wear full uniform … but the training team came in and wore plain clothes and that contradicted what the council wanted from the contract. They were telling us to hide in bushes and to hide behind cars and to make sure no one saw our logo,” he said.

He added: “I believe this company is profiting from poverty. Some people would trawl around estates or out of town centres and know where to target because it was easier. Some people would target certain age groups, like over-60s, as they were law abiding and if issued with a fixed penalty notice they would pay it.”

A few months ago a woman told Grimsby Live that she was followed for 15 minutes, including into a local pub, by five Kingdom officers for dropping a cigarette on Victoria Street. A member of North East Lincolnshire council, where the alleged conduct took place, strongly rejected the claim that enforcement officers harass people.

In north Wales, an action group has been set up with 8,523 members who have run campaigns and marches against Kingdom. Members want to see the problem of littering and dog-fouling dealt with in-house.

Across England and Wales, seven councils – including Liverpool, Anglesey, and Flintshire – no longer outsource the enforcement of minor environmental and antisocial behaviour fines to the firm. But at least 14 local authorities still have contracts with Kingdom Services Group.

Liverpool dropped the company in November after complaints. The city’s mayor, Joe Anderson, said: “65% [of Kingdom officers] were operating in the city centre and 65% of fines were for disgarding cigarette butts. I am not saying that’s right, there are toxins in cigarettes that do environmental damage, but people were saying officers were lying in wait for people to come out of office buildings to smoke and fining them. That’s not what we are about.”

Kingdom said that it had ended a number of the contracts itself “due to commercial decisions it has taken”.

Kingdom Services Group still lists Liverpool city council as a customer on its website, and also says that it operates a range of payment schemes guaranteeing no cost to the local authority.

“Kingdom’s costs are recovered by the fixed penalty notices (FPNs) we issue and, with our average 75% payment rate, which has resulted in £3.3m for local authorities during the last 12 months,” the website states.

The company has denied it operates a bonus scheme. It says it pays its staff “an hourly rate together with an allowance for strict adherence to quality and procedures, which ensures controls are followed”. This is done to maintain “high standards”, it says.

The company says FPNs are determined by legislation that also provides strict guidelines as to what constitutes an offence. It said that Kingdom officers are required to operate within them and do so in an effort to protect environmental standards.

Lara ten Caten, a legal Officer at Liberty, said: “These orders are overwhelmingly used in poorer areas – outsourcing enforcement to a company that is just in it to make as much money as possible is not just callous but irresponsible and a failure of governance.”