Large numbers of cleaners in the commercial cleaning industry are experiencing underpayment or non-payment of their wages, a detailed report into pay and conditions of office cleaning staff has revealed.

Commercial cleaning staff, usually women and migrant workers, complained of being frequently mistreated by employers, who often ignore obligations to provide holiday and sick pay, according to the new study by the Equality and Human Rights Commission (EHRC), The Invisible Workforce: Outsourcing and Employment in the Cleaning Sector.

The report also cites disturbing examples of abuse of cleaning staff by agency bosses, and notes that often cleaners feel unable to report problems for fear of losing their jobs. It cites examples of workers who were sacked for complaining about not being paid in full and on time.

Many cleaners said they felt that both their employers and staff at the client firms did not treat them with appropriate dignity and respect. Many spoke of being "invisible" and "the lowest of the low", and of being spoken to rudely.

The study, which focuses on the non-domestic sector, looked at cleaners working in offices and in the health, retail, transport and leisure sectors.

The wholesale outsourcing of office cleaning to agencies from the 1970s (when most organisations had in-house cleaners) has led to downward pressure on wages and working conditions. "Contracts often place cleaning firms under enormous pressure to deliver a high-quality service at the lowest cost possible. This often has a negative impact on employment practices, affecting pay, the intensity of work, job security, training and working hours," the report finds.

The cleaning sector contributes more than £8bn to the British economy, and consists of around half a million workers, the report states. The workforce is comparatively old, with those between 45 and 54 years accounting for 26%, and those over 55 accounting for a further 22%. In London, migrant workers make up as much as 44% of the total workforce. Migrant workers' poor command of English and lack of knowledge about employment rights makes them particularly vulnerable to mistreatment.

Cleaners interviewed reported pay rates for private sector contracts ranging between £5 and £7.50 an hour, indicating that some are paid below the national minimum wage of £6.31. While the study conceded that those who were underpaid were the minority, researchers found "a significant number of workers are not paid in full, or do not receive the holiday or sick pay they are entitled to".

Cleaning staff complained about unrealistic workloads and "work intensification". "They felt that clients and cleaning firms regularly underestimated the amount of hours or staff needed to do a particular job. As a result cleaning operatives had to work at a very fast pace for hours without a break, and this added to stress," the report says.

"When we complain, they punish us with more work. When you work at full speed, [so] you can't work any more, then people don't complain," a Bolivian woman employed by a cleaning firm, cleaning offices and student accommodation, said. The report states: "Migrant workers did not seek redress as they often were unaware of their employment rights. They did not know that they were entitled to holiday leave or maternity pay, and in some cases were misinformed by their employers."

The most commonly cited problem was the lack of recognition of the role cleaners played in enabling other organisations to function. "Many workers felt they were looked down on, spoken to rudely, treated differently and worse than others, harassed and abused by the client, supervisers, or members of the public. Some workers felt hurt, isolated and unhappy because of this," researchers found.

"There is a misconception … that we are 'thick cleaners' and intellectually inferior. The work is often hard and the lack of thanks and appreciation tends to make one somewhat insular leaving one deflated and ignored … Little do people realise how much they need us," one British cleaner, working in healthcare, told researchers.

Some cleaners said they had nowhere to take a break, and were denied access to staff canteens. Others said cleaning staff had to eat in cupboards full of mops, buckets and cleaning chemicals.

"We get no respect or support from our supervisers and the ward staff shout at us. This is because we are not regarded as human beings. We are afraid to complain, in case we are sacked," a female Czech agency cleaner, working in healthcare, said.

The EHRC has proposed a number of measures to improve conditions for cleaners. It is calling on clients to pay the living wage (£7.65 an hour and £8.80 in London) but public sector employers said significant government budget cuts had a big impact on the price they were able to pay for cleaning contracts.

The EHRC also suggests cleaners' work conditions can be enhanced if clients switch to daytime cleaning, allowing cleaners to work better, more sociable hours. It highlighted the importance of greeting and thanking cleaning staff, and asking employers to consider what action they can take to ensure that cleaners are treated with the same dignity and respect as their own workforce.

The equality and human rights commissioner Caroline Waters said: "Fairness, dignity and respect are values we all share. Yet the commission uncovered some disturbing evidence of the absence of these in the treatment of many cleaning workers by supervisors, clients and the public. Cleaners are largely invisible, despite the cleaning workforce numbering nearly half a million people … They may be bullied by supervisors, have problems with their pay, and have excessive workloads. This can often be linked to a focus on reducing costs at the expense of good practice in employment and contracting."

The TUC general secretary, Frances O'Grady, said: "This report shines a light on British businesses' dirty secret – the appalling conditions faced by those who clean their offices and workplaces.

"Agencies who employ cleaners – as well as the millions of businesses who use their services – have a duty of care to ensure cleaning staff are treated fairly. Living wage contracts are a great place to start."

'He thought he could ask for whatever he wanted'

Narcisa Calderon, 43, a migrant worker from Ecuador, says her poor English made her vulnerable to exploitation when she came to the UK 12 years ago and began working as a cleaner in London.

"Before I came here I was a secretary in an office. It was a much better job. Many of the cleaners I've worked with here have been engineers or doctors or had better careers in their own country. The only job I could find here was cleaning because of the language barrier."

She worked first as a night-time contract cleaner in a solicitors' firm, 10pm to 6am. It was exhausting work, but during their rest hour they were not allowed to shut their eyes. One night when they were having their allocated break, a manager came in and banged the table.

"I understood the action, but I didn't speak enough English to understand what he was saying. Someone told me later he said: 'Bitch, no one can sleep in this area.'"

An office cleaner works at night. Photograph: Alamy

She left that job because of the pressure, and moved to a different job in the City, but left after a few months because she was expected to clean 80 toilets in a short shift, and the work was beyond her. "I got depression, and I had to leave that job."

When she transferred to a new agency, she was confronted by an area manager who used to promise her extra shifts in exchange for sex. "He had the power so he thought he could ask for whatever he wanted."

She declined but did not complain because she was anxious not to lose her job. Initially, she was confused by unexplained pay deductions, which she felt unable to challenge because she was not confident enough in English.

Things have become easier for her during the 12 years she has spent in this country as her grasp of English has improved. She now works for the London Cleaning Agency, where employers are "kind and fair".

But she is still surprised by the rudeness of office staff in the buildings she cleans.

"The clients – there is a lack of respect, they treat us like we aren't human. We say hello, they ignore us, like we are not there. It makes you feel awful. In the beginning it surprised me; it is a shock to be ignored.

"I thought: 'What is the difference between them and us? We are cleaners but we are still people.' Now I am not shocked. Some people are busy and some people are just rude."