A £1.3 billion investment in the Post Office has been announced, with the aim of "transforming" thousands of branches, including longer opening hours.
Around 6,000 post offices will be converted into more modern branches over the next three years in what was described as a "once-in-a-generation" opportunity to revitalise the network.
The Government repeated its pledge that no more post offices will close, but the postal workers' union warned that making wholesale changes to the network was a "major gamble" and involved "downgrading" some branches.
The Post Office said all its 11,800 branches will benefit directly or indirectly from the investment, which is subject to European Union state aid clearance.
Almost half of post offices will convert to a "new operating model", covering either larger branches, which will offer extended opening hours, or "local style" offices, where a post office counter will operate inside a convenience-type store, also with longer hours.
Subpostmasters who convert to a main style branch will be offered up to £45,000 to help the change, while those opting for a local style will have access to investment of up to £10,000.
The programme was aimed at making the post office more self-sustaining and less dependent on subsidies, said the post office, adding that the number of weekly visits to offices had fallen from 28 million to 20 million in the past 12 years.
Paula Vennells, chief executive officer of Post Office Ltd, said: "This is a once-in-a-generation opportunity for the Post Office - a chance to revitalise the network and allow it to meet the needs of 21st century customers and communities. This major investment will help Post Office Ltd address changing customer needs by revitalising the network, with extended opening hours and a better experience for our customers."
Postal Affairs Minister Norman Lamb said: "Post Offices are at the heart of many of our communities, providing much needed services. This Government has made a commitment that there will be no closure programme and invested £1.34 billion to secure the long term future of the Post Office.
"However, change is needed to secure the long term future of post offices. Pilot schemes are showing the new 'locals' and 'mains' models are popular with consumers and subpostmasters, so I am pleased that Post Office Ltd are putting in place another 6,000 of these post offices over the next three years. The new models will make the network more financially viable and give customers what they want when they want it."