More than 160 High Street post offices could be shut down or sold off under plans being considered by Royal Mail.
The plans could affect up to 270 high street post offices
The review of 560 Crown post offices which are directly run by Royal Mail, will involve unions and consumers.
Up to 20 could be shut down, while over 100 more could be moved to less costly sites or franchised to independent operators, a Royal Mail source said.
The Communication Workers Union said the government should support Crown post offices' "valuable" service.
"Post Office Ltd hasn't reached a decision about the future of the 560 Crown post offices, but we confirmed a shortfall of £70m in the last year," a Royal Mail spokesman said.
"Discussions are ongoing and options include closures and offering local business people the chance to take over the offices."
A Royal Mail source told the BBC that the number of Crown post offices could fall below 400 but there were likely to be no more than 20 outright closures.
FEARS OVER CLOSURES
Elderly and disabled people could struggle to get to branches further away
Many people forced to rely on and pay for public transport to access branches
Longer queues and poorer facilities due to increased pressure on branches
Fall in standard of local delivery services
Damage to local businesses and reduced community spirit
Franchising deals could involve supermarkets providing post office counters, as already happens in some communities.
Liberal Democrat trade and industry spokesman Malcolm Bruce said: "We need to know under what criteria Royal Mail would close these offices - those that have the lowest business value, those that provide the least in terms of community service, or those with the greatest commercial value when sold."
Any Crown post office closures would be in addition to the ongoing closure of up to 3,000 urban post offices across the UK.
More than 1,500 sites have been closed since this programme launched in October 2002.
Postwatch chairman Peter Carr said those closures had been agreed on the basis that the Crown post offices would remain, and called the latest development "rather surprising".
"The risk is that these offices will be closed and there will be a big hole in the particular urban area where access to a post office will not be adequate," he said.
"Post Office Ltd will have to make sure that there is adequate provision in the area."
Andy Furey, of the Communication Workers Union, said: "We are very keen to safeguard the future of our members and a valuable public service that is well-loved.
"Rural sub post offices have quite rightly been supported by the government - all the Communication Workers Union is asking is that the same consideration be given to Crown post offices."
The government has invested £450m over three years, due to run out in 2006, to try to halt the closure of rural post offices.
The government announced in September a further £300m to be available from 2006 to 2008.
Crown post offices are commonly found in busy high streets and could command vast sums if sold off for commercial use.
Former sites in Glasgow's George Square and at Clapham Common in London have already been put up for sale, and other branches in major cities could follow.