Post office bosses were accused of "blackmail" after a leaked letter showed that they have warned postmasters they will lose thousands of pounds in compensation if they do not stick to the officially approved script on closures.
In a highly contentious move, undercover staff will be sent around the country to check that postmasters are using the official "lines to take" when customers ask about the Government's plans to close 2,500 post offices.
The woman in charge of the closure programme spelt out the plans in a letter to postmasters, which was leaked to the Conservatives.
The letter from Sue Huggins, the director responsible for the so-called Network Change Programme, was accompanied by a two-page crib-sheet spelling out the "key messages" they must deliver if people ask about the closures.
Miss Huggins made clear that postmasters whose offices are selected for closure could lose compensation packages - which are worth about £60,000 - if they did not co-operate.
"During the Programme, representatives of the Post Office Limited will visit branches at random, on an anonymous basis, in order to ensure these 'key messages' are being delivered in an accurate and professional manner," said Miss Huggins, in the letter sent to thousands of postmasters.
"Subject to state aid clearance, any compensation package 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."