One in five workers in the UK is paid less than required for a basic standard of living, a report has claimed.
The proportion is much higher among waiters and bar staff, at up to 90% of workers, the research for accountants KPMG suggested.
It claimed 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."