Former Labour leader Lord Kinnock has warned the UK government it risks being seen as treating the Welsh as "second rate" by cutting the number of Welsh MPs.
He said the proposed cut would lead to "the feeling of being downgraded, marginalised and dispensable".
The proposals would see the number of Welsh MPs reduced from 40 to 30.
The UK government has previously said Wales was "significantly over-represented".
Lord Kinnock was speaking in the House of Lords as peers launched an eleventh-hour bid to limit the reduction in numbers.
The UK government wants to cut the number of seats in the House of Commons from 650 to 600 and to equalise the number of voters in each constituency.
These plans would see the number of Welsh MPs reduced from 40 to 30.
Lord Kinnock said: "When those feelings are nourished, sometimes by mischievous politics but also by economic and social circumstances, they develop a life of their own."
He warned of "the risks that are being taken with the political psychology and the patriotic feelings of the people of Wales by their being treated so arbitrarily, so disproportionately and so outrageously".
Cross-bench peer Lord Rowe-Beddoe said: "We are dealing with one of the most unfair pieces of legislation, as far as Wales is concerned, that I could possibly conceive."
But the Conservative former minister Lord Hamilton told him the union was threatened by devolution.
"Once you start the process of devolution it becomes a ratchet with more powers then transferred, in the case of Wales, to the devolved assembly," he said.
Cross-bencher Lord Elystan-Morgan said: "The case for the special treatment of Wales is its special, individual status as a nation.
"If you deny that case, you deny the essential meaning and significance of that nation."
The Advocate General for Scotland, Liberal Democrat Lord Wallace, said ministers were not proposing less representation for Wales than for other parts of the UK.
Lord Wallace said: "This bill provides that the value of a vote in Wales will be the same as the value of a vote in England, Scotland and Northern Ireland, within a 10% range of tolerance."
Prime Minister David Cameron has dismissed concerns about the size of some of the new seats, privately arguing that in Australia MPs need to fly around their constituencies.
But Labour peer Lord Lipsey argued that the huge size of Brecon and Radnorshire, which is 90 minutes by road from one end to the other, means it should be a special case.
His argument was rejected by the UK government, leaving Lord Lipsey to complain: "The very sheep are baa-ing their dismay at the words they have just heard in their fields in the Brecon Beacons."
The Parliamentary Voting Systems and Constituencies Bill, which contains the changes, needs to become law next week if the proposed referendum on the voting system is to be held on May 5.