An estimated £20m is donated to charity each year via Postal Order, but until now charities have been unable to claim tax relief on most of these funds because they did not have donors' permission to do so.
The new Postal Orders, which were released by the Post Office last week, have been designed with a Gift Aid declaration on the reverse that charity supporters can sign to make their donations tax-efficient.
If the whole £20m were to be claimed via Gift Aid, an extra £5.6m could be made available.
The Postal Orders, which were developed in consultation with The Giving Campaign, have been launched in time for Christmas, traditionally the most lucrative period for charitable giving.
"I am delighted that the Post Office is going the extra mile to support charities," said Philip Mind, director of The Giving Campaign. "This will unlock extra millions for good causes and raise the profile of Gift Aid considerably."
The Postal Orders will be promoted inside post offices across the country, and the Post Office is encouraging charities to promote the change to their supporters.
The initiative is the latest in a series of innovations intended to revitalise the postal order. It follows the launch of the gift postal order, for the use of postal orders as presents, and a commemorative postal order to mark the Queen's golden jubilee.
The Post Office said the use of postal orders has remained static for many years, and are used by online shoppers.