All UK employees can now ask for flexible working hours under new rules.
The right to ask will be available to some 20 million workers with more than six months' service, having formerly been restricted to carers and parents.
The government said that it expects the new rules to be of particular interest to older workers who are coming up to retirement and to youngsters who want to take advantage of additional training while in work.
Employers will now be required to consider all requests in a "reasonable manner" and must notify a worker of a decision within three months of a written request being made.
However, the new rules don't mean employers are obligated to agree. There remain eight "sound" business reasons for rejecting a request. These include the burden of additional costs on the business and a detrimental effect on the business's ability to meet customer demand and performance.
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."
Susannah Clements at the Chartered Institute of Personal Development, added: "The extension of the right to request flexible working to all employees is recognition of its growing importance to both employers and individuals.
"Employers increasingly recognise the strong business case for flexible working, including enhanced employee engagement and the attraction and retention of a more diverse workforce."