A new model of providing postal services in stores or petrol stations should be improved after research revealed problems such as inaccurate product and pricing advice, according to a consumer group.
Consumer Focus said the so-called Post Office Locals, which are increasingly being opened, have potential benefits such as longer opening hours and greater convenience.
But a study, based on mystery shopping at over 100 branches trialling the model, found that more than one in six customers could not access a service which should have been available, privacy was often found to be poor, while cash payments could not be made in some cases.
Mike O'Connor, chief executive of Consumer Focus, said: "Changes are needed to put the post office network on a more sustainable financial footing. Communities rely on their local post office, so everything possible must be done to make sure that all elements of Post Office Locals are working well for customers, given the planned national roll out of such major changes.
"This report shows a mixed picture and that further work is needed to make sure the millions of customers who will be affected receive a good quality and reliable service. Some aspects of the new locals model could be great news for consumers. Everyone will welcome longer opening hours, but they would also expect most core post office services to be available."
Communication Workers Union leader Billy Hayes said: "The report confirms our concerns that the locals model threatens the post office network and the services which communities rely on. The report draws attention to the failings of the model, but we are less optimistic that it can be made to work than the watchdog. The lack of proof that financially the model is viable for postmasters is disappointing as it leaves this central question unanswered.
"High quality post office services are unlikely to be delivered by a retailer whose primary focus is the quick and efficient sale of his or her retail products - whether that's petrol, cigarettes or groceries. Their eye will be on the bottom line and staff costs will be crucial to this, especially with the ambition of longer opening hours."
A Post Office spokesman said: "We welcome this report and its acknowledgement that Post Office Local style branches offer many positives for improving the customer experience, with greater convenience and extended opening hours, which were welcomed by the vast majority of consumers.
"With the local pilot ongoing, there are some improvements we can make but as the report highlights, we are already addressing some of these issues. The local format remains an important part of a significant overall investment programme to sustain and modernise the post office network and to grow the business."
Shadow postal affairs minister Ian Murray said: "I fear that the roll-out of the Post Office local model may be being rushed without giving full consideration on the impact to consumers. Some aspects of the local model are very welcome but the rush to short-term cost reduction in the local model may lead to longer term decline of the network and we must ensure Post Office Limited use the Consumer Focus research to make changes for the better."