The decision to allow RBS customers access to their accounts through the Post Office was announced last November as an alternative to introducing a Post Bank scheme. RBS said at the time that the programme would be implemented as "soon as possible", but according to a Post Office spokesman it won't be available until the end of September.
Geoff Roger, who runs Cranbourne Post Office in Dorset, said that many of his customers were keen to use the service and had been frustrated by the delay as they didn't have easy access to their money. "Cash machines are not readily available in rural areas so it makes sense for post offices to welcome customers from all banks, rather than just a few of them," he said.
"It would benefit many people if RBS customers could use post offices like mine to access cash and make deposits, as well as bringing in more customers."
Those who bank with Barclays, Clydesdale, Co-operative and Lloyds banks can already access accounts through the network.
Meanwhile, the Welsh Assembly has announced that grants worth a total of £849,658 will be given to 87 post offices across the country.
The funding, made available under the Post Office Diversification Fund, is designed to help post offices expand their business and offer new services to their communities.
Corwen sub post office in Clwyd South received £20,100 to install a coffee shop, CCTV and computer facilities.
The Post Office Diversification Fund was set up in December 2108 and £3.5m has been awarded to 229 post offices throughout Wales. The fund is now closed to applications, with an evaluation report on its impact due in the autumn.