Wales could lose up to eight MPs after a law was passed to redraw constituency boundaries.
The number of MPs across the UK will remain at 650 but the number of Welsh seats in House of Commons could fall from 40 to 32.
The UK Government said equally sized constituencies is a “sensible policy that will make our elections fairer”.
But Plaid Cymru said Wales would lose out more than any other nation of the UK or region of England.
The Boundary Commission, which is responsible for drawing constituency boundaries, is generally required to propose constituencies whose electorates vary in size by no more than plus or minus 5% of the average.
But it was announced in June that the island constituency of Ynys Môn would gain “protected status”, meaning it could not be lost as a UK Parliamentary seat after the boundary review.
The boundary commission will start the constituency review in January to determine the average number of electors that will be in each constituency and the new boundaries.
The new electoral maps will undergo three separate consultations before the final proposals are presented to the Commons by 1 July 2023.
The UK government’s Constitution Minister, Chloe Smith MP, said: “Every voter deserves to have confidence that their vote counts the same, no matter where it is cast. This assurance is long overdue and today’s Act delivers exactly that.
“Up-to-date, more equally sized constituencies is a sensible policy that will make our elections fairer, ensuring that people from all four nations of the UK have equal representation in Parliament.”
Plaid Cymru’s Parliamentary Leader, Liz Saville Roberts MP, said: “It’s hardly a coincidence that Wales’ voices in London are being reduced at the same time as the Tories are clawing back our powers.”
Welsh parliament committees had previously accused the UK government of threatening to “undermine devolution” and “profoundly” limit the Senedd’s powers after Brexit following proposals to replace EU rules for businesses.
The UK Internal Market Bill is supposed to ensure trade continues smoothly after 1 January
“Wales will lose out more than any other nation of the UK or region of England as a result of this legislation,” said Ms Saville Roberts.
“Our nation deserves proper democratic representation – that means urgently granting more powers to our Senedd.”