The CWU says the service faces an "ongoing dispute" with members if it is not prepared to change its plans.
Post Office workers will strike later this month in a row over branch closures, jobs and pensions.
The Communication Workers Union said thousands of members will walk out on 15 September after voting overwhelmingly to take industrial action.
Staff face 2,000 job cuts this year as well as the closure of their pension scheme, the union says.
General secretary Dave Ward said the Post Office was at "crisis point" and urged the Government to step in.
"The Post Office is relentlessly pursuing a programme of cuts that will mean a further 2,000 job losses, staff being left tens of thousands of pounds worse off in retirement and the privatisation of its flagship branches," he said.
"We are making a simple demand. The Government needs to pause the cuts, convene a summit of key stakeholders in the industry and work out a strategy that gives employees and the public confidence that the Post Office has a future.
"It needs a serious plan to grow revenues in areas like financial services."
He said other countries had brought in "hugely successful Post Banks" and that there was no reason "why our Post Office should be the world leader in managed decline".
In a ballot, 83% of members who voted were in favour of walking out.
Unions say the Post Office needs a serious plan to grow revenues
Deputy General Secretary Terry Pullinger said next week's 24-hour strike would be the first day of industrial action.
He said the Post Office faced an ongoing dispute with members if it was not prepared to change its position.
Mr Pullinger claimed the Post Office was offering staff "thousands of pounds not to go on strike" and was threatening to make CWU reps redundant.
"It is incredible that a publicly owned company is behaving this way and the Government has got to make clear that this is completely unacceptable," he said.
Before the ballot, the Post Office said comments from the CWU about its performance were "misleading" and said they would "cause undue concern for customers and employees".
Post Office network and sales director Kevin Gilliland said the service had halved losses in 2015/16 and was making "steady progress" in reducing costs to the taxpayer.
"We are making steady progress to modernise the UK's biggest retail network," he said.
"These changes are needed to make our services better for customers and ensure that Post Office branches thrive at the heart of communities for future generations.
"We want to work with our unions to create the Post Office network that our customers need for the future, and will pursue this discussion through our scheduled talks."