The figure represents 23% of all employees, up by 1% over the past year, the research by professional services firm KPMG found.
The report was published ahead of new figures being published on Monday for the hourly rate, which are expected to show an increase from the current £7.85 an hour and £9.15 in London.
The current figures are well above the national minimum wage and more than the new living wage of £7.20 an hour for over 25-year-olds the Government has announced will come into force next April.
KPMG said its research revealed that the proportion of workers earning less than the voluntary living wage had risen for three years in a row.
The data also showed a "worrying trend" of part-time, female and young workers being most likely to earn below the figure.
Mike Kelly, of KPMG, said: "The past year has seen some notable achievements, with 2,000 employers, including more than a quarter of the FTSE 100, now accredited by the Living Wage Foundation.
"Awareness of the issue has also increased, with more than three out of four of the general public in the know about what the living wage is.
"With the cost of living still high, the squeeze on household finances remains acute, meaning that the reality for many is that they are forced to live hand-to-mouth. The figures released today show that there is still more to be done if we are to eradicate in-work poverty.
"For some time it was easy for businesses to hide behind the argument that increased wages hit their bottom line, but there is ample evidence to suggest the opposite, in the shape of higher retention and higher productivity.
"It may not be possible for every business, but it is certainly not impossible to explore the feasibility of paying the living wage."
Shadow business secretary Angela Eagle said: "We need to see more better-paid, high-skilled jobs which pay a living wage, but these figures show that progress is slipping.
"While the Tories claim to be on the side of working people, these new figures show that the number of people being paid less than the living wage is up since last year.
"At the same time the Tories are making life harder for those on low and middle incomes as they cut tax credits and hit families with a work penalty - taking on average £1,300 out of the pockets of up to three million working families. People are working harder than ever, but will rightly feel that the Government is working against them."
A Government spokeswoman said: "Britain deserves a pay rise and from April 2016 the new national living qage will give a direct boost in wages for 2.7 million people across the country, meaning a full-time worker will earn over £4,800 more by 2020 and with up to 6 million expected to see their wages rise by 2020 as the knock-on effects are felt higher up the earnings scale.
"Treasury analysis shows that women and those based outside London and the South East will be the biggest winners when the new national living wage comes into force.
"We are determined to move to a higher wage economy and welcome any employers who choose to pay above the national living wage."
The UK's 10 best-paid jobs
Six million workers 'paid less than voluntary living wage'
Flying high literally as well as metaphorically, pilots top the salary scale this year. There are various routes into the job, from university courses to airlines' own training schemes; private training will set you back as much as £50,000 to £60,000.
"I most enjoy the variety of the job, working as a team, to a schedule, with the responsibility of safely transporting hundreds of people to their destination. I would advise aspiring pilots to thoroughly research training routes, understand the serious financial outlay required with no guarantee of a job after training and to consider other routes such as flight instruction."
Not all CEOs are making millions - but they're doing pretty nicely on average. There's no clear route into the job; an MBA can help, but many bosses have worked their way up from the bottom and many simply started their own firm.
"There is an old saying, it's very lonely at the top, which is usually the case for a CEO. You're the leader and you're responsible for everything, meaning you take the credit for the successes and feel the most pain for the companies' failures."
Most air traffic controllers train through National Air Traffic Services (NATS), which has programmes for college leavers who have studied at BTEC ND/HNC/HND level, university students on a sandwich year and graduates. NATS says the most important attribute for acceptance on a course is natural aptitude.
"Much of the time, the job is routine and capable of being done by many people. What you are paid for, however, is your ability to make the right choices and decisions under extreme pressure."
The high pay reflects both the heavy workload and the length of training required - a five-year degree course in medicine, a two-year foundation programme of general training and then another three years specialist training.
"You will get paid well and (despite recent changes!) you will get a very good pension. You will, however, work hard for your money with less recognition than previous years."
This is a job where most people work their way up, although qualifications like the Chartered Institute of Marketing's Professional Diploma in Marketing or the Institute of Direct and Digital Marketing's Diploma in Direct and Interactive Marketing can help. You'll probably need three or four years' experience before you can move into management.
"It's great working in such a varied job - there's something new every day. I enjoy the travel, too. But it's fast-paced, very hard work, and there's a lot of technical data analysis work involved."
It's not surprising that IT and telecoms directors earn good money. The job requires both business and technical skills, and the stakes are high: smoothly-running IT systems are crucial for most businesses and the cost of getting things wrong is high.
"It's a high-pressure job, and the hours can be unbelievably long, but it's extremely rewarding when a project goes well. The biggest problems are staying on top of changes in technology and being clear about what's actually achievable within a budget."
Financial managers and directors need to be qualified accountants and, particularly in larger organisations, have strong general business skills. And the job carries huge levels of responsibility, from keeping the company in good health financially to making sure it complies with the law.
"What I enjoy about the job is the way it's central to the business, the way I'm involved in creating long-term business plans. Compliance [with regulations] can be a bit of a pain; it takes a lot of hard work."
Until recently, the only way to become a senior officer was to work your way up through the ranks. Each police force sets its own recruitment process and selection policy, though there's a basic national framework for evaluating competency. This year, though, the College of Policing has started appointing superintendents from outside the force - so you might be in with a chance.
"I love the opportunity to take on a vast array of roles which would never be possible in a civilian organisation and to progress through a clearly defined promotion structure, should I prove to be good enough. The training I have received has been quite brilliant, enhancing my life skills as well as my ability to do my job well."
If your vision of a bank manager is of a middle-aged man in a panelled office dictating stern letters to his secretary, think again. It's now about managing a team and, especially, attracting new customers and monitoring branch performance. There are two ways to get to be one: either through a bank's graduate scheme, or by starting off in a customer service role and working your way up.
"For me, what's great about working in a branch is that anyone could be a branch manager, you don't have to have a certain skill set. Having a background in customer service, whether it's in a bank or with another retailer, certainly helps. Some other skills that would help are organisation, patience, tolerance and empathy. After 27 years I can genuinely say that that not only do I love coming to work every day, I have also made lifelong friends here. For me, it's not just a nine-to-five job."
Once you're qualified as a teacher, it's a question of working your way up by gaining extra responsibilities - by becoming a head of department, a head of year, or a specialist in something like special educational needs. At head teacher level, the job is far less about teaching itself, and far more about managing staff and budgets and creating a productive, disciplined learning environment.
"It's a common complaint that teaching is all about performance targets these days, and there is some truth in that. It's a question of balance, though. The most rewarding part of the job is knowing that you're bringing the best out of every single child by creating an environment where they feel safe and motivated to learn."