More than 1,000 post offices have shut or been put up for sale this year, with some not showing up in official closure lists, according to new research today.
The Communication Workers Union said there was a "growing trend" of closures in 2010 which it warned would get worse if the Government pressed ahead with controversial plans to privatise the Royal Mail.
A study by the union showed that 162 post offices were listed as "long-term temporary closures" this year and over 900 were put up for sale,
Many are likely to stay closed indefinitely and those that do re-open are liable to provide a diminished service, such as an outreach van with no access to financial services, said the union.
The CWU said almost 10% of the entire network was currently up for sale, warning they could also be lost to the "temporary closure loophole".
Many subpostmasters are retiring or leaving the business because of the low levels of revenue generated in sub-offices, and in many cases it is difficult to find alternative premises or service providers, according to the CWU.
General secretary Billy Hayes said: "Post offices have been closing at an alarming rate this year, but didn't register because they are misleadingly classed as temporary closures.
"We're seriously concerned about empty government rhetoric on avoiding post office closures. These promises are contradicted by government taking business away from the Post Office and by the planned privatisation of Royal Mail.
"Post Offices rely on work generated by mails business and most would be unsustainable if this was removed. Many of the proposals in the Postal Services Bill, including privatisation of Royal Mail and splitting the postal group apart, seriously threaten mails business arrangements for the Post Office.
"With uncertainty hanging over Royal Mail and the post office network, it's no surprise that buyers are hard to find. The government must change its approach or face the decimation of our cherished post office network. Who's going to buy a business which stands to lose a substantial part of its income?"
Ministers have pledged there will be no repeat of the closure programme under the previous Labour Government, arguing that the plan to privatise the Royal Mail will safeguard the branch network.
A Post Office spokesman said: "The Government has already said there is no programme of closures.
"The Post Office has the largest network in the UK and there will always be an element of turnover. The figures quoted by the CWU simply reflect the number of temporary closures and the number of branches that change hands.
"The important thing to remember is that the majority of branches reopen when there is a temporary closure.
"In fact, more than 200 branches change hands every three months and historically, up to 10% of the network of more than 11,500 branches will change ownership annually."
A Department for Business spokesman said: "This is scaremongering from the CWU. The reality is that at least 200 branches change hands every quarter - there is nothing unusual about this figure."