One in five workers in the UK is paid less than required for a basic standard of living, a report has said.
The proportion is much higher among waiters and bar staff, at up to 90% of workers, the research for accountants KPMG suggested.
It said that nearly five million people failed to command the living wage - a pay packet that enabled a basic standard of living.
The rate stands at £8.30 an hour in London and £7.20 in the rest of the UK.
This rate is voluntary, unlike the National Minimum Wage - the amount that employers must pay by law, which is set at £6.19 an hour for those aged 21 and over.
"Times are difficult for many people, but of course those on the lowest pay are suffering the most," said Marianne Fallon, head of corporate affairs at KPMG, which has itself signed up to pay the living wage.
"Paying a living wage makes a huge difference to the individuals and their families and yet does not actually cost an employer much more.
"Tackling in-work poverty is also vital if we are to enable more people to improve their life prospects and increase social mobility in this country."
The report suggested that Northern Ireland had the highest proportion of people earning below the living wage, at 24% of workers, followed by Wales at 23%.
The lowest levels were in London and the South East of England, both at 16%, it said. In terms of total numbers, London, the North West of England and the South East of England had the most.
When looking at sectors of employers, some 90% of bar staff and 85% of waiters and waitresses failed to get as much as the living wage.
Some 780,000 sales and retail assistants were not paid to living wage level, the highest total of any group of employees, the report suggested.
Frances O'Grady, the incoming general secretary of the TUC, said: "It is shocking that in this day and age, one in five workers is still earning less than is needed to maintain a decent standard of living.
"The living wage is not a luxury, and means that low-paid workers do not have to make tough choices over whether they can afford the everyday things that most of us take for granted, such as their fuel bill or a winter coat for their children.
"Many more employers could afford to adopt the living wage, and we hope that many more decide to pay it in the coming months. Now more than ever is the time for employers to put an end to poverty pay."
But Mike Cherry, policy chairman for the Federation of Small Businesses, said: "Every employer would want to be as reasonable as they possibly can, but in the current economic climate it is not going to be possible for those sectors that have traditionally been unable to pay the national minimum wage."
He said rent and rates were becoming more expensive, and so were energy costs, so the living wage was an aspiration but not affordable for some employers.
He added that the market would determine what was affordable.
A separate report by the CBI said that employers have needed to take a cautious approach to employment and pay given the economic climate, and this is set to continue.
The group said that there would be pay restraint over the next six months, but this was designed to protect employment.
However, one 23-year-old care worker told the BBC News website that life was tough financially - even when on the living wage.
She said that the cost of petrol, when driving between the homes of the people she cared for, took a big chunk out of her pay which totals £7.21 an hour.
"It would be nice to have enough so I am not worried about paying rent every month and only having £100 left to spend," she said.
"I would like to live comfortably rather than struggling from pay cheque to pay cheque."
The government is considering whether to push on with plans to end national pay bargaining and introduce a system of regional or local pay rates.
However, a group of 60 academics have warned, in a letter to The Times newspaper, that any new system could widen inequalities between different parts of the UK.
The group claimed that there was "no convincing evidence" that regional pay would boost local economies, and that it could reduce consumer spending. They argued that would depress pay for public sector workers outside London and the South East of England.
This follows a campaign by unions to stop any introduction of regional pay rates for public sector workers.