It is 10am on a Saturday morning and Whitechapel High Street in London is even busier than usual. A long, slow-moving stream of people snakes past the lines of stalls selling saris and Tupperware, and a circle of dancers has formed around a boombox playing Bob Marley. If it were not for the sea of signs saying “No racism in the NHS”, “Slippery Serco” and “We’re cleaners, not dirt” bobbing to a constant chorus of “Low pay, no way”, the scene could be a carnival. In fact it is the fifth day of the biggest cleaners’ strike in British history, due to end next week, which has seen 750 of London’s lowest-paid workers take action against one of the country’s wealthiest corporations.

The high spirits are suddenly punctured by outbursts of rage as the crowd gathers outside the Royal London hospital, one of four hospitals where staff are involved in the strike against the outsourcing giant Serco. The company was recently awarded the £600m domestic services contract for Barts Health NHS trust, which runs the Royal London alongside St Bartholomew’s, Mile End, Whipps Cross and Newham University hospitals.

One of the strikers is Adwoa Bema, 52, who came to London from Ghana in the 80s and worked cash-in-hand cleaning jobs until she got a job at the Royal London 12 years ago. “I’ve cleaned all my life. I know what it’s like to work hard, but this is just too much. Some days I feel like I’ll drop dead if I carry on.” From Monday to Friday, Bema wakes up at 4am for a 5am start at her first job cleaning an office. When the staff start to arrive at 7.30am, she heads to Whitechapel to start an eight-hour shift. “I still live in my overdraft – we all do. All we can afford to do is eat and pay rent and bills.”

When Serco took over the Barts Health contract in April, it agreed to pay all workers the London living wage of £9.75 an hour, but due to rapidly increasing living costs, many still have to take on a second job. Unite members, employed by Serco as domestic staff, porters and security workers at the trust, want a 30p an hour wage increase; that has been rejected.

Serco’s contract director, Phil Mitchell, says that the living wage increase “resulted in an average increase in pay of 3.5% for over 140 permanent staff”. He adds that for staff on higher salaries, “we have protected all their terms and conditions and offered a pay increase for this year which is in line with other NHS colleagues”.

But pay is only part of it. Since Serco took over, Bema says, she has experienced a huge increase in her workload. “I am now doing the job of three people. I have people following me telling me to clean more beds, more rooms. I’ve started doing the jobs of healthcare assistants and have received no extra training. My colleagues working in the kitchen are doing the same. Many of us have extra jobs and we are so tired and cannot do our jobs properly. I hurt everywhere when I get home and all I do now is eat and sleep.” Serco says that cleaners no longer have to serve meals, as they did previously, and that their roles are now focused “exclusively on cleaning duties” within single wards.

Most of the workers on strike are women from east and west Africa. Like Bema, many had never been part of any kind of industrial action until the morning of 4 April, only three days after Serco had started. That day, cleaners were greeted by a letter saying they were to have no more paid tea breaks. Mary Agyei, who has worked at the hospital for nine years, decided to round up 140 cleaners in the fifth-floor canteen. She told them they were to take their break together. “We shouldn’t be ashamed of wanting to take a break. We work hard. We need it. Lots of people were nervous they would be sacked, but if we were all together they couldn’t get rid of us.” She was right, and by Friday the breaks were back. Serco has since apologised – and says the change had been “initiated by a local manager”.

Agyei is now one of the main representatives of the workers and steps up to make a speech. “We will not be bullied any more,” she shouts into her microphone. “Now is the time to stand together!”

Seeing the power of this collective action, a stream of workers began to join Unite. Then, on 17 July, workers began a seven-day strike. This led to talks between Serco and Unite in mid-July, but no agreement was reached. “The offer [made by Serco] was totally inadequate, divisive and aimed only at a small segment of the membership,” says Len Hockey, a senior strike representative, who works as a porter. Serco claims it made a “serious” offer that would have boosted the pay of 800 employees.

“As you can see, this isn’t just about money, this is about dignity and respect,” says Agyei. “I always speak my mind. I now know what acting together can do for us. I can’t sit down and let people bully my friends and colleagues. They’re making people sad and they’re making people ill. Our new supervisors intimidate us – they always pick on the weaker ones. They are getting us to do other people’s jobs and we are fed up.” Serco has denied all these claims.

Agyei will be 60 in January and has just had her second grandchild. “We work so hard, but we have hardly any time or money and it’s hard for us to see family.”

Florence Kwao has been working as a cleaner for eight years. “We are being pushed and we can’t go on,” she says. “My body hurts and I have waist pain every evening because I just work so hard. On top of all this, it’s hard to do our job well. We just haven’t the time to do it properly.”

In a hospital, the job of a cleaner is essential. Florence Nightingale said that hygiene in the hospital is the essence of basic care, and this remains the case 150 years later. It takes time and careful, skilled work to carry out the hygiene measures required to keep bacteria such as MRSA at bay.

According to a 2014 report published by the Equality and Human Rights Commission, the non-domestic cleaning sector contributes more than £8bn to the economy each year and comprises “a largely invisible workforce of around half a million people”, predominantly women, a high proportion of them black and minority ethnic. Many cleaners start out on informal arrangements that leave them feeling insecure and unable to take industrial action.

Agyei arrived from Ghana 25 years ago and started working at the Royal London in 2008 as an agency worker. According to Matthew Dore-Weeks, a senior organiser at Unite, many private companies favour agency workers “because they are insecure and find it very hard to organise together, and can be more easily exploited or hired and fired. Agency workers can join trade unions, but they cannot take part in strike action because it is very difficult for us to legally ballot these workers as we did with Mary and the other Serco workers. It’s a huge problem and it is why agency workers and zero-hour contracts are on the rise.” According to Dore-Weeks, many of the cleaners at the trust will not be able to strike as Serco has increased the number of agency workers. Serco says the new agency workers are to cover training, in addition to employees, and that it has moved 49 members of staff from zero-hours contracts to guaranteed hours.

Caroline Robinson, director of Flex, an organisation that fights labour exploitation, says: “Many migrant women feel that they are unable to take action as for various reasons they don’t have the information or help they need to do so.” Robinson says that many migrant women feel they are unable to take action. “The fact that women migrant workers are organising in this sector when all odds are stacked against them is brave and important,” she says.

Many of the cleaners at Barts Trust say they have been emboldened by the success of other cleaning strikes, notably the strike by cleaners at the London School of Economics against the private contractor Noonan. That action was led predominantly by women from Jamaica represented by United Voices of the World (UVW), a grassroots union that focuses on improving conditions for migrant workers. “The majority of workers we represent are migrants working in London’s low-wage, outsourced economy” says UVW’s general secretary, Petros Elia. “There is a lot to be positive about. We are seeing a lot of workers who would never normally join unions now wanting to take action for the first time. Things are slowly beginning to change.”

He adds: “The belief you can win is more prevalent now and collective action really does work. These battles are important. It’s about deep-rooted inequality, which is shown in the awful treatment of cleaners, which is so prevalent in Britain.”

Offering his support to the strike is the Labour leader, Jeremy Corbyn. In a statement sent to the Guardian, he put the blame squarely on NHS privatisation. “These cleaners do vital work for the NHS, but work for a private company because of privatisation and outsourcing. This company made a profit of £82m last year, but despite this is refusing them a 30p pay rise.

“By taking collective action to improve their conditions, these workers are showing that when the many work together, they can take on the powerful few who are holding them back.”

At the back of the crowd is one cleaner who did not want to be named. She is in her 60s and moved to London from Pakistan 30 years ago. She has been working as a cleaner all her working life, but now finds herself unable to retire due to high living costs, but struggling to carry on working due to poor health. Despite chanting along with the crowd, she is not on strike, as she is an agency worker and will soon have to return to work after her lunch break. “I wanted to come along for a bit,” she says quietly. Despite the rising number of cleaners joining unions and speaking up for the first time, many voices will still go unheard in Britain’s cleaning industry.