Struggling post offices are facing an "extremely uncertain future" because of falling incomes and a lack of new work, according to a survey.
Research carried out by the National Federation of Subpostmasters (NFSP) found a large number of branches are under serious threat, as well as a many local retail outlets. The NFSP said the survey showed a "clear failure" in the Government's plan to turn post offices into a "front office" for government services, which was announced in 2010.
Figures based on responses to 743 questionnaires revealed just 17% of subpostmasters said they could see a strong future for their business while 51% said they had seen their Post Office income decrease in the past year.
The research found Post Office overheads increased for 70% of subpostmasters while staff costs rose for 48% of those who responded.
George Thomson, general secretary of the NFSP, said: "There is now little doubt that the Government's promise of no more planned closures was hollow and that the failure to make full use of the network will lead to mass closures through the back door.
"Subpostmasters are angry over the shocking failure of both Post Office financial services and government services to bring in any material increase in revenue for the vast majority of sub post offices, leaving many with an extremely uncertain future.
"Urgent action must be taken to avert a major financial crisis in the post office network and further post office closures which will leave the network unable to compete with competitors at a time of huge change. We are looking at all our options to ensure vital changes are made."
The NFSP said the survey revealed few new services had been introduced since 2010 and most of the financial and government services were yielding little or no income for the vast majority of subpostmasters.
It said less than 10% of subpostmasters receive any income from many of the key Post Office financial services, such as credit cards, insurance for homes, cars and vans, and life assurance, while 70% reported an increase in overheads and 48% reported an increase in staff costs.
A spokesman from the Department for Business, Innovation & Skills said: "The Government has committed £1.34 billion to maintain the network of at least 11,500 post offices across the country and there is not, and will not, be a programme of closures. Government funding will allow for the modernisation of around half of the UK's post offices, allowing them to compete in a changing retail market. This change will take time, but we are committed to the future of the Post Office and helping it to compete in a changing retail market."