National Federation of Subpostmasters general secretary George Thomson predicted that the money would go to waste if links with the Royal Mail weren't maintained and new contracts failed to be awarded.
"Unless government delivers on these issues, the funding package risks achieving nothing more than creating the best refurbished, but emptiest, branches in Europe," he said.
The Rural Shops Alliance (RSA) welcomed the funding, but urged upgrading its computer systems a priority. "This is a once-in-a-generation chance to put the Post Office on a sustainable footing," said RSA chief executive Ken Parsons. "The key need is to bring the company's computer systems into the 21st century, enabling the network to provide government services and compete more effectively for commercial contracts."
Business secretary Vince Cable said he hoped the investment, to be spread over four years, would "reverse the years of decline and secure the network's future".
He added that the package would prevent future large-scale post office closures. "It will also allow the Post Office to invest, improve its offer and win new revenue streams," he said.
The government is to publish a policy statement detailing its plans, including using it to provide more government services.