Plans to close 2,500 post offices will come under renewed scrutiny after Labour rebels came within 11 votes of forcing a halt to it.
A Conservative proposal to suspend the programme pending further consultation was defeated by just 20 votes, as 19 Labour MPs revolted, slashing the Government's majority by two thirds.
Labour critics said the willingness of so many colleagues to back a Tory motion proved the depth of upset over the issue and called for a rethink.
The revolt came as ministers gave the green light to local councils to help take over post offices planned for closure - at their own cost.
Post offices are losing around £3.5 million a week and there are four million fewer customers than two years ago.
When the latest post office closure programme was announced, the Government pointed out that it had invested £2 billion in the network between 1997 and 2006 and would commit an extra £1.7 billion to the year 2011.
Rebel MP John McDonnell said: "The Government has always underestimated the strength of anger on the Labour backbenches against the privatisation and cuts in this essential public service."
He said the vote was "a huge embarrassment to the Government and shows that a large number of Labour MPs are even prepared to support a Tory motion to demonstrate their concern".
The Tories had urged the revolt, pointing out that 90 Labour MPs, including seven Cabinet ministers, had campaigned against proposed closures in their own constituencies.
Shadow business secretary Alan Duncan said the "astonishing" vote showed his party had won the argument.
