The right to ask employers for flexible work arrangements have been extended to 4.5 million parents of children up to the age of 16.
The change means millions more employees have won the right to request flexible working conditions and takes the total number of people in Britain able to request changes to the traditional working day to more than 10 million.
Employees now have the right to request to work part-time, from home or make other variations to their working hours, including job-sharing, compressed hours or school term-time hours.
Business leaders have warned that extending flexible working rights will be damaging to small businesses.
Business secretary Lord Mandelson's attempts to delay the plan to help businesses cope with the recession were overruled.
Small business groups have pointed out that small and medium-sized employers have had to deal with a host of new employment legislation introduced by Labour and are ill-equipped to cope with more in the current economic climate.
“The smallest businesses need their key members of staff to be present in the workplace wherever possible," said Phil Orford, chief executive of the Forum of Private Business.
"Many are already struggling with mounting costs and increasing bureaucracy, and this will only add to their difficulties."
While employers can decline any flexible working requests which could harm their business, they are obliged to give one of eight valid business reasons for doing so and it is expected that around 95% of requests will be granted.
Employees can appeal internally against a refusal, and then go to an employment tribunal, where they can be awarded up to eight weeks' pay.
The CBI says the existing right to request flexible working is working well but had urged the Government to delay the extension until October.
The British Chambers of Commerce said the cost of regulations to business introduced by Labour since 1998 has risen by £10bn to £77bn.
© Crimson Business Ltd. 2009