One in five sub-post offices could close under moves to trial new ways of offering postal services, such as in retail outlets and garages, a new report has said.
Consumer Focus said the so-called Post Office Local offered benefits such as longer opening hours but could also lead to fewer services being available.
The consumer watchdog said a survey of 1,000 people showed most welcomed the convenience and extended opening hours, especially in the evening and at weekends.
But half of those polled had to use another post office because services were not available, while a similar number said privacy was poor, especially for financial transactions. More than a third said the quality of service was average or poor and there were problems making cash and benefit withdrawals.
Consumer Focus said any "significant shift" to Post Office Locals could see up to one in five dedicated sub-post offices close.
Spokesman Andy Burrows said: "Before Post Office Locals are rolled out nationwide, it is vital that the Post Office delivers a model that meets its customer's needs, both now and in the future. Our research highlights some plus points but also many warning lights which must be urgently addressed."
"We recognise that the Post Office must change if it is to be successful in the future. PO Locals have the potential to modernise the network, but the design and implementation must be absolutely right for their customers. To make the network sustainable they must also be able to benefit from future revenue streams including banking transactions and the lucrative market for parcel drop-off and collection, which is being fuelled by online shopping."
A Post Office spokesman said: "The Government and Post Office Ltd have made it absolutely clear that there will be no further programmes of closure. The Post Office Local concept is simply a way of providing post offices that offer more convenient opening hours to customers and enable the operator to make a good return on running the post office within his or her store."
A Department for Business spokesman said: "The Post Office Local model is an important part of the Government's strategy for the future of the Post Office, and will enable the company to offer what customers really want - convenient locations and longer opening hours - and to compete effectively in today's fast-changing retail environment."
Billy Hayes, general secretary of the Communication Workers Union, said: "Locals are failing. Government and the Post Office need to listen to customers about what works and what doesn't. We're told that financial services will play a growing role in Post Office revenue, but Locals clearly can't deliver that."