Millions of employees will have the right to request flexible working from today under new measures the Government believes will particularly benefit older people.
The right has only been available for carers, or people who look after children, but is being extended to all employees.
Ministers said flexible working helps people balance their work with responsibilities, keeping more people in long-term employment and enabling companies to retain staff.
The Government said it expected the new right will be of particular interest to older workers who want to work differently as they approach retirement and to young people who may want take up additional training or learning while they work.
Any request will have to be considered in a "reasonable manner" by employers, while the process is being made simpler.
Deputy Prime Minister Nick Clegg said: "Modern businesses know that flexible working boosts productivity and staff morale, and helps them keep their top talent so that they can grow. It's about time we brought working practices bang up to date with the needs, and choices, of our modern families.
"Today is a crucial milestone in how we can help people balance their family life with work and caring responsibilities, and from next year, shared parental leave will allow mums and dads to be able to choose how they care for their new-born in those first precious months."
The conciliation service Acas has published a code of practice to help employers understand the extension to the right and how to process requests.
Chairman Brendan Barber said: "Our experience from working with thousands of employers is that flexible working is both good for business and employees.
"The new code will help employers handle flexible working requests in a reasonable manner and fit their specific circumstances."
TUC general secretary Frances O'Grady said: "It's not just parents and carers who can benefit from flexible working. This sensible and modern approach to work is something that can improve the lives of everyone.
"Now, thanks to this long overdue change in the law, employees of all ages will be able to ask their boss to alter the way they work, regardless of whether they have dependents or caring responsibilities.
"If they have an employer who gets why flexible working makes sense, workers who want to take time out to train, volunteer in a local community project, or simply avoid travelling at rush hour will now be able to transform their lives.
"But those with old-fashioned bosses who expect all staff to stick to the same rigid hours day in day out and always be in the office won't be so lucky. Employers will still find it all too easy to block any requests for greater flexibility.
"Unfortunately the right to request is only the right to ask nicely. There is nothing to stop employers saying no."
John Allan, chairman of the Federation of Small Businesses, said: "Small businesses often work in close-knit teams and are flexible by nature. Many small businesses therefore already offer flexible working and recognise the benefits of doing so, such as boosting productivity and staff morale, without the need for a 'right to request.
"A recent FSB survey found that four in five small firms currently offer flexible working or would consider offering it if asked."