Proposals for a huge shake-up of the UK parliamentary map in Wales have provoked criticism for threatening to break up tight-knit valley communities in the south and creating vast, unwieldy rural constituencies. The Boundary Commission for Wales admitted that the proposed changes, announced on Wednesday, were significant” but said the decision of the UK parliament to reduce the number of Welsh MPs in Westminster from 40 to 32 had meant it had to re-draw the map for all but one of the nation’s constituencies. Polling experts said that Labour, the Tories and Plaid Cymru all stood to lose seats at the next general election under the changes. But they said that Labour’s strength in Wales means it is likely to be the biggest loser. Martin Baxter, CEO of polling analysts Electoral Calculus, said: “All the parties are likely to lose some seats in Wales because of the shrinking number of seats. Labour will probably remain the largest party in Wales with these new boundaries, but it will be a tiny bit harder for Keir Starmer to get a majority in Westminster at the next election.” The former Welsh secretary Peter Hain called the proposals “an anti Labour, anti valleys charter”. Hain added: “Communities in each of these valleys have different histories, different identities, including on the Welsh language. By their very nature, valley communities are linked and don’t easily connect with neighbouring valleys.” He said already large constituencies in mid, west and north Wales would become “monster ones”. “Mega constituencies will only alienate voters from the people they elect to represent them,” he said. Some constituencies would get new names while others would be fully absorbed into neighbouring constituencies. Among the famous names that would be lost is the constituency of Ogmore, which has been held by Labour for its entire 101-year history. Another Labour stronghold, Caerphilly, would go and a new constituency, Newport west and Caerphilly, formed. There would no longer be a Neath constituency but Swansea east and Neath is proposed. Six constituencies would have an area of more than 1,000km2 and one, Brecon and Radnor in mid Wales, would cover more than 3,000km2. In the north, the constituency of Arfon, currently held by Plaid Cymru, would vanish from the map. In the west, Ceredigion, another Plaid seat, would go and a bigger constituency, Ceredigion Preseli is created, taking in parts of Pembrokeshire, currently a Tory area. Under the new rules, each constituency must contain between 69,724 and 77,062 electors. The only exception is Ynys Môn (Anglesey), which is allowed to remain as it is. As well as considering voter numbers, the commission factored in geography, such as lakes, rivers, and mountains and current local authority boundaries. The commission said it also considered local ties, such as shared history and culture. It is inviting comment on its proposals as it launches an initial eight-week consultation period. The secretary to the commission, Shereen Williams, said: “We’ve had to propose significant changes due to the reduction in the number of Welsh constituencies and that’s presented a particular challenge.” Jess Blair, director of the Electoral Reform Society Cymru, said: “With a cut to Welsh MPs at Westminster we need to think seriously about whether it’s finally time to increase the size of the Senedd in Cardiff Bay. Our Welsh parliament is dwarfed in size by a number of local councils across Wales.” The changes do not affect Welsh parliament boundaries.