Thirteen post offices in mid Wales are the latest to be earmarked for possible closure as part of Post Office's UK-wide review.
The central Wales region currently has 166 Post Office branches
The changes are needed to make the network viable, says Post Office Ltd.
The proposals also include establishing 24 outreach service points to replace some branches.
Conservatives and Liberal Democrats criticised the plans which could see branches in Montgomeryshire, Ceredigion and Brecon and Radnorshire areas close.
The announcement follows the decision to close 29 branches in Cardiff, Bridgend, Vale of Glamorgan and valleys.
A decision on 25 threatened branches across the Newport and Gwent Valleys has yet to be made.
The proposals for mid Wales will leave 153 post offices - five branches in towns and 148 in rural areas.
Llandrindod Wells: Tremont Road
Llandrindod Wells: Ridgebourne, Grosvenor Road
Brecon: Westend Service Station, Church Street, Llanfaes
Brecon: Pendre Spar store, Bryndewinton
Blaenannerch: mobile service by BT telephone box, A487
Lampeter: Abermeurig mobile service
Lampeter: Talsarn mobile service, B4337 Main Road
Swansea: Cwmgiedd, 50-52 Heol Giedd, Ystradgynlais
Newtown 4 Oakfield Terrace, Llanidloes Road
Newtown: Garth Owen
The region currently has 166 post offices.
The company says more than 98.8% of the region's 200,000 population will either see no change or will remain within one mile, by road, of an alternative branch.
The new outreach service points will provide post office services for a designated number of hours per week in that particular area and could be located within a mobile van, a host shop or village hall.
In very small communities, a home service could be provided whereby a sub postmaster delivers a reduced range of products and services which can be ordered over the telephone.
Under the plans, 89.3% of the area's population will retain their existing post office branch, said Post Office Ltd.
Kirsty Williams AM and Roger Williams MP have petitions opposed to closures
Where there is a change, 9.5% would have access to post office services within one mile by road distance to the nearest alternative branch.
Post Office Ltd said it had consulted with watchdog Postwatch and all the local authorities within the area to come up with the plans.
It said it had considered geographical, public transport and local population and economic issues.
But the Liberal Democrat MP and AM for the Brecon and Radnorshire area - Roger Williams and Kirsty Williams - said the announcement was "shattering" and "devastating".
Mr Williams said the pair would be meeting all the affected town and community councils in order to build up individual cases against each of the proposed closures.
Meanwhile, Conservatives said the mobile outreach units would result in a far more limited service.
"People will be forced to fit around a timetable for accessing Post Office services," said Nick Bourne, AM for mid and west Wales and leader of the Welsh Conservatives.
"This is incredibly restrictive for those who work and the elderly.
"While the use of outreach services like mobile units is to be welcomed, it is a very poor substitute for the real thing."
The normal consultation period of six weeks has been extended by three weeks and three days to allow for the local elections and the Easter holidays.
The public can have their say on the matter until 19 May.
Steve Geraty, Post Office Ltd's network development manager for Wales, said: "Taking the decision to close any post office branch is always very difficult and we know will cause concern to many of our customers.
"We want to ensure that everyone who uses, relies on, or has any concern with post office services is both fully aware of the proposed changes and able to give views on them."
The plans are part of a wider restructuring of Post Office services throughout the UK.
Altogether, 2,500 post offices - a fifth of those left - are to close by 2009.