A Midland postmaster warned he will be forced to close unless the Government backtracks over controversial reforms to pensions and benefits.
Lakhvir Randhawa, of Tamworth, Staffordshire, was one of thousands of subpostmasters who gathered in London for a rally yesterday.
They warned that plans to axe Post Office Card Accounts could spell the death of the post office network.
The National Federation of Subpostmasters handed a petition of four million to 10 Downing Street - claiming it was the largest ever domestic petition handed to a Prime Minister.
But pressed in the House of Commons yesterday, Tony Blair insisted there could be no further subsidies for post offices, which already loses £200 million a year.
Conservative leader David Cameron focused on the issue in the House of Commons, arguing that card accounts, used by millions to access pensions and benefits,were a "vital" income stream for post offices.
The Department of Work and Pensions is to axe the accounts, currently used by 97,500 people in Birmingham and Solihull, by 2010. It wants the money paid into bank accounts instead.
Mr Randhaw said his store, which is also a newsagent, might remain open in some form if card accounts were abolished but the post office counter would have to close.
He said: "It is a massive threat. If you end the card account, running a post office b ecomes unaffordable, because it is a major stream of income."
He has run his shop, a family business near the centre of Tamworth, for 18 years.
Colin Baker, general secretary of the The National Federation of Subpostmasters, said: "The 28 million customers who use the Post Office every week are confused as to what the Government wants of the network and of the people who run it.
"We believe that the time is now overdue for Ministers to decide."
The number of post offices has fallen from 18,393 in 1999 to 14,376 in 2005.
Almost 400 MPs have signed a Parliamentary motion calling for card accounts to be saved, including Birmingham MPs Lynne Jones (Lab Selly Oak), Richard Burden (Lab Northfield), Clare Short (Lab Ladywood), John Hemming (Lib Dem Perry Barr), Andrew Mitchell (Con Sutton Coldfield) and Roger Godsiff (Lab Sparkbrook & Small Heath).
Speaking in the House of Commons, Mr Blair pledged to consider all options in the Government's review of the Post Office network, but ruled out further subsidies.
The Prime Minister said the Government had invested about £2 billion in the network, with current subsidies running at £150 million a year.
He told MPs that a key problem facing the Post Office was people's increasing use of bank accounts.
Tory leader David Cameron said that if decisions were not taken urgently there would be no Post Office network left to protect