Opponents of the Government's controversial plans to privatise the Royal Mail have claimed there are signs of a "mini-rebellion" by coalition MPs before the Bill reaches its final stages in the Commons on Wednesday.
Postal affairs minister Ed Davey said the Royal Mail and the Post Office were at a "crossroads", adding that the sell-off plans promised the service a "brighter future".
Westminster sources said there was growing support for amendments being put forward to the Postal Services Bill, including moves to make sure there was an inter-service agreement between the Royal Mail and the Post Office.
The Communication Workers Union (CWU) has warned that selling off the Royal Mail arm of the business will hit post offices and could lead to a fresh wave of branch closures.
Mr Davey said: "We won't repeat the previous government's post office closure programmes. The Post Office is not for sale. Instead we are providing £1.34 billion of new funding and developing new reasons for customers to keep coming through the door.
"Royal Mail has a multibillion-pound pension deficit, is faced with rapidly declining letter volumes, needs much greater efficiency and has an urgent need for capital at a time when there are huge constraints on the public purse.
"A visit to a sorting centre just before Christmas brought home to me once again the huge task that faces Royal Mail. Staff were working incredibly hard to ensure that endless rows of presents and parcels bought online were ready for delivery. The digital age is presenting the company with a different set of challenges, and only with fresh ideas, modernisation and more investment can Royal Mail really adapt and thrive in this new market.
"The Post Office also has to rise to these challenges and make the network even more attractive and convenient for shoppers - expanding new services for customers and small businesses using their local post office to drop off and collect parcels. I know how much people up and down the country value these great institutions and the vital services they provide - the Government is determined to secure the future of both."
CWU general secretary Billy Hayes said: "There are genuine concerns from the public, small businesses and mail users about the Government's Bill. Whilst we and opposition MPs are opposing the sale, there is also an opportunity for Government MPs to address the genuine concerns of mail users. Amendments to protect post offices and the universal service have been tabled.
"We want to see robust protection for post offices in the form of an inter-business agreement which is currently missing from the legislation. Without protection, the Post Office stands to lose a third of its income, putting offices across the country at risk of closure. Many MPs have tabled important and sensible amendments aimed at protecting postal services. We hope that the Government takes these on board and doesn't see it as a political game of saving face."