Every employee now has the right to request flexible working hours after the government extended the right previously reserved for carers and those looking after children.
As part of the right, employees with at least six months' service can expect their request to be considered "in a reasonable manner" by employers.
The Department for Business, Innovation and Skills said 20 million people now had the right to ask to work flexibly.
The Government expects the extension of flexible working rights to be of particular interest to older workers approaching retirement and to young workers looking for additional training while they work.
The Chartered Institute of Personnel Development said the change in the law was recognition of the growing importance of flexible working for both employees and employers.
"Employers increasingly recognise the strong business case for flexible working, including enhanced employee engagement and the attraction and retention of a more diverse workforce," said the CIPD's chief executive Susannah Clements.