The change is being made to implement a revised EU Parental Leave Directive, which was originally due for implementation in March 2012.
However, the Government decided to take advantage of a period of 12 months' grace available under the Directive to defer making the change.
Each child who receives disability allowance, the allocation of unpaid parental leave will increase to 18 weeks up until its 18th birthday.
Parents will also have the right to take the leave in respect of each child they have, however they will be restricted to four weeks of unpaid leave each year.
Reasons for taking such leave can include problems with the child's schooling, changes to childcare arrangements, attending medical appointments and the need to spend more time with their child.
The Government hopes these changes will aid people with their work-life balance as well as encouraging parents in their career development.
Audrey Williams, partner at law firm, Eversheds said: "In reality, the change is unlikely to make a significant difference to employers. The fact that the leave is unpaid means that take up has not, to date at least, been great.
"Changes to maternity and paternity leave, both recent and proposed, mean that fathers are able to take more significant periods of time off following the birth, or adoption, of a child, and recourse to the unpaid parental leave provisions is infrequent."
Williams added: "Much further into the future, the Government plans to relax the age restrictions on taking unpaid parental leave.
It proposes that, from 2015, all parents will have until the child's 18th birthday to take their leave entitlement, creating a much wider window of opportunity.
"However, this proposal has not been included in the Children and Families Bill which is currently on its way through Parliament."
