Shadow chancellor John McDonnell has told hospital workers their strike action for better pay is "justified", branding their treatment exploitation.
He was speaking at a rally at the Royal London Hospital in east London in support of hundreds of striking low-paid cleaners, porters and security guards employed by Serco at Barts Health NHS Trust.
On Saturday the workers were on the fifth day of a seven-day strike, having already taken three days of industrial action between July 4 and July 7.
Mr McDonnell said: "You have my 100% support in this dispute.
"Jeremy Corbyn has gone down to the Tolpuddle celebrations this weekend, so he can't be here. But I bring you a message of solidarity from Jeremy Corbyn, the leader of the Labour Party.
"I want you to know that we are with you.
"The reason you have my 100% support is because your cause is just.
"It is not much to ask, is it? In the fifth-richest country in the world, in the richest city in the fifth-richest country in the world, to earn a decent wage where you can feed your children and house your family.
"It's not much to ask.
"It's not much to ask when you know the company that has privatised these services are making multimillion profits every year.
"And their directors and their managers are lining their pockets at your expense.
"Do you know, there's a word for this? And it's 'exploitation'. And that's what we are seeing.
"To be frank, they're not bothered about the workers they employ and they're not bothered about the service they provide.
"This strike is justified."
Mr McDonnell spoke to hundreds of people who had gathered at the east London hospital waving banners bearing messages including: "Serco Pay Up" and "Low Pay, No Way!"
The campaigners were in good spirits at the almost carnival-like protest, dancing and singing before the speeches began.
They have also warned Barts they are prepared to stage a 14-day stoppage, starting on July 25.
Workers are protesting over low pay and poor treatment, and asking for a pay increase of 30p per hour.
The dispute involves workers at Whipps Cross University Hospital, Royal London Hospital, St Bartholomew's Hospital and Mile End Hospital.
Mr McDonnell concluded: "We are human beings, we have the right to live a decent life. We have the right to have a real wage.
"We have the right to put a decent roof over our heads. We have the right to be able to feed our kids.
"This dispute is about basic human rights. And we will support you throughout, until we win."
Earlier in the week, Phil Mitchell, Serco's contract director, said: "We believe strongly in ensuring our dedicated team are rewarded fairly, which is why we agreed with the Trust to pay all our team members a minimum of the London Living Wage from day one.
"This resulted in an increase in pay for over 230 team members and benefited over 110 permanent staff by an average of 3.5%. For those team members on higher salaries, we have offered a pay increase for this year which is in line with that for NHS staff.
"We're determined to ensure that Unite's action does not impact on patients. We have robust plans in place aimed at ensuring we can continue to support the Trust's hospitals to operate as normal during the planned action."