Efforts to safeguard postal services are at a standstill because of the Post Office's failure to co-operate.
Barnet Council accused the Post Office of refusing to disclose financial information on the branches facing closure and the subsidies available to keep them open.
And council leaders say the information is vital in determining any future course of action to save them.
The impasse follows months of failed communications between the two bodies after it was announced that eight post offices in the borough would close as part of the Government's cull of 2,500 branches nationwide.
The Childs Hill branch in Cricklewood Lane was the first of the eight post offices to close on Tuesday, with the others shutting within two weeks.
Council leader Mike Freer said: "We need to look at the financial circumstances of each branch and see if we can pick up the tab.
"It's like trying to push water up a hill with a fork. They are being unhelpful and slow to respond and, by the time we get to a position where we could make a decision, it might be too late."
A Post Office spokesman refuted suggestions that the company was being obstructive and said positive discussions were going on with other councils.
"We are now in a position to provide financial information to Barnet Council since it signed the non-disclosure agreement that all other local authorities talking with us have signed," he added.
"The problem we face is that Barnet Council has declined so far to specify which offices it is interested in funding. We therefore can't know which financial information to provide."
The council wrote to the Post Office in April when the closures were first proposed, asking it to share information on the floorspace and subsidies available for all branches in the borough earmarked for closure.
But last week the Post Office replied claiming it did not have the data available, which leaves a question mark over whether it will be possible to maintain the services.
Mr Freer said: "Post Office Ltd has spun time and again that it will assist genuine proposals to re-provide branch services. We put ours on the table and it has told us, after two whole months, it will not help.
"It is not interested in working with anyone to save postal services, it wants the branches cut. Full stop."
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