Labour has admitted the elderly in rural areas would be hit hardest in plans to halve the total number of post offices.
It says 'nearly half' of all those nationwide are no longer profitable and subsidies are unsustainable, meaning nearly 6,000 of the 11,500 risk being scrapped.
Part-time ones which open a few days a week or share premises with other businesses are also threatened. Village post offices are most at risk, especially if they can't afford to open full-time.
The Government also suggests it could redefine what is meant by a 'branch'. This means 'drop and collect' boxes could count as post offices.
The threats are in the fine print of Labour's Future of Post Office consultation, which closes tomorrow.
The Tories have created a petition railing against the changes, with shadow post office minister Harriett Baldwin saying: 'Post offices are a precious part of our critical national infrastructure.
'Every community must have access to essential services like cash, parcels and communications.'
In the consultation, the Government recognises that post offices are important community hubs that provide the most 'social value' for elderly people. It also admits those in rural areas depend on them more.
Many postmasters are still fighting for compensation after being persecuted in the Horizon faulty computers scandal.
A Department for Business spokesman said last night: 'It is right to consider a range of options to secure the organisation's long-term future but our preference is to keep the overall size and shape of the network the same.'