The right to demand flexible working is to be extended to an extra 4.5million parents, it was revealed yesterday.

They will be able to ask to work part-time, work from home, or make a range of other variations to their hours until their children reach 16.

Gordon Brown believes that chaining staff to their desks for the traditional 9pm to 5pm working day is helping to breed unruly children.

However business leaders said the shake-up, which should become law by next April, could prove a "nightmare" for small firms.

The plan was previewed in Wednesday's draft Queen's Speech, but the full details were confirmed yesterday.

The right to request flexible working patterns is already available to parents of children under six.

The Prime Minister denied that giving it to millions more would impose any "burden" on British business.

"We are a society determined that our children grow up in a way that is a credit not just to our families but also to society as a whole," he said.

Harriet Harman, the Women and Equalities Minister, added: "Parents want both to earn a living and do the best they can in bringing up their children, but need more flexibility at work.

"We've already built a strong foundation of support for families, with the right to request flexible working for parents of children up to the age of six, as well as improved maternity and paternity rights.

"But, as any parent knows, the demands of parental responsibility don't end at the age of six, which is why we are going to extend the right to request flexible work to parents with older children."

The Government is also to introduce a law giving new rights to agency staff following a campaign by trades unions.

And workers are to get a new right to ask for time off to go on training courses so they can improve their skills.

Employers will be legally obliged to seriously consider requests for training, but will be able to refuse if they can produce a good business reason.

But the pledges set the Government on a collision course with business leaders, who complain they have had to contend with a flood of new employment rights introduced by Labour and are ill-prepared to cope with more.

Employment experts also warned of increasing resentment among childless workers at the special rights being given to those with families.

Mike Emmott, of the Chartered Institute of Personnel and Development, said: "The danger with ever larger groups of people entitled to request flexible working, and a smaller number not entitled to do so, is that divisions will grow up in the workplace."

Katherine Rake, director of equality campaign group the Fawcett Society, said: "We warmly welcome the Government's decision to extend the right to request flexible working to parents of children up to 16 years old.

"But today's announcements are not the wholesale reform women urgently need. The continued restriction of flexible working rights to a particular group means it remains the 'mummy track', with those that take it up stigmatised as committed to their job."

The Federation of Small Businesses warned that extension of the right to request flexible working together with new rules on agency workers could create huge problems for small companies.

Its employment chairman, Alan Tyrrell, said: "The announcement puts small businesses in an impossible position.

"You can't have an extension of flexible working and at the same time clamp down on the means by which many small businesses cope with it, which is often through temporary workers.

"The current flexible working regime seems to be working, but the Government should be cautious about extending it too far, which could be damaging to small businesses and as a result the millions of people they employ."

Susan Anderson, director of human resources at the CBI, said: "The right to request flexible working has worked well precisely because it is a 'right to request', not a 'right to have'.

"But extending the right to another four and a half million parents is a big step and the Government must give firms enough time to prepare, particularly small firms.

"The extension should come into effect no sooner than October 2009 - not next April - as the process to make it law could easily run into the New Year."

Since 2003, 3.6million parents of children under six and disabled children under 18 have had the right to request flexible working patterns.

It has since been extended to 2.65million carers of adults.

While employers can decline the requests, they have to give one of eight valid business reasons for doing so and 95 per cent are 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 Conservatives accused Mr Brown of stealing the flexible working idea from them.

Theresa May, shadow minister for women, said: "We want to go further than these recommendations and last year we made a clear commitment to extend flexible working rights to all parents with children under the age of 18."