The Post Office has apologised for threatening postmasters with financial penalties for failing to give official explanations for branch closures.
Plans to close 2,500 post offices were announced in May
A letter had warned that undercover officials would visit post offices to ensure that "key messages" were given.
In a move condemned by Conservatives as "blackmail", it added postmasters whose branches closed could lose compensation deals if they did not fall into line.
A spokesman for the Post Office said the letter had been wrongly sent out.
The letter, from Sue Huggins, the Post Office's director of the Network Change Programme, said the anonymous inspectors would check that the plans to close 2,500 branches were being presented to customers in an "accurate and professional manner".
It continued: "Subject to state aid clearance, any compensation offered to you if your branch is selected for closure shall be subject to you having complied, and continuing to comply, with this obligation up to the date of closure."
Ms Huggins adds that the details of the programme should be kept confidential. But the letter went on: "We do appreciate that customers may ask questions that you might find difficult to answer."
A Post Office spokesman said the letter should not have been sent out as it contained "factual inaccuracies".
He added: "We apologise for this and will be writing to sub-postmasters next week confirming that.
"We would like to make it clear that we will not be doing anonymous testing, that's not going to happen - it was never going to happen.
The Conservatives condemned the approach.
Shadow Business, Enterprise and Regulatory Reform Secretary, Alan Duncan, said: "It is not appropriate in a free country for a national employer like the Post Office to use the tactics of the secret police to demand Maoist conformity to their line.
"Explaining their view is one thing, but to go around spying on their branches and employees is quite sick, and will only make things worse."
George Thomson, general secretary of the National Federation of Sub-postmasters, said: "The Federation accepts that this was an error of judgment by Post Office Limited and we are confident that Post Office Limited will apologise to sub-postmasters in due course."
In May, plans to close 2,500 post offices were announced by the government after Alistair Darling, then Trade and Industry Secretary, said that the network could not be sustained with losses at £4m a week.