Government proposals for English votes for English laws (EVEL) are "ill thought out" and could lead to Britain becoming more like the stricken eurozone, former chancellor Alistair Darling has warned.
The Labour stalwart criticised plans put forward by Commons Leader William Hague to resolve the perceived imbalance of power between England and the devolved nations of the UK and change the way laws pass through Parliament.
Mr Darling, who led the successful campaign to keep Scotland in the UK, said he was concerned that the current proposals could undermine the way the UK works and resemble a "bolt on on the fag end" of the coalition's time in office.
Prime Minister David Cameron announced plans to proceed with EVEL the morning after Scotland voted to stay in the union despite a 5am phone call from Mr Darling warning him not to raise the issue so soon after the independence referendum.
The former chancellor said he was concerned about the way plans to deliver EVEL were progressing.
He told BBC Radio 4's PM programme: "I looked at the stuff that the Government has published on English votes and it really is pretty ill thought out and raises and awful lot of very good questions and the proceeds to answer none of them."
He went on: "I am concerned that we get ourselves into a situation where in trying to answer what is known as the West Lothian question about which MPs can vote on health and education, we undermine the economic and political union that makes the UK work.
"It's the oldest political and economic union of its size in the world.
"Once you start saying MPs from parts of the kingdom can't vote on Budget matters, you'll end up like the eurozone, and nobody voted for that.
"So I think that there are some very profound issues here."
Mr Darling said the EVEL plans could result in a "profound threat" to the union and urged political leaders to come up with a better solution.
He told the programme: "I'll put it this way - almost inadvertently I could see the Conservative Party climbing into the same bed as the Scottish National Party and ending up with a profound threat to that United Kingdom."
He went on: "Another clear principle in my mind is will this work under stress?
"Because remember what happened in the eurozone, it was never going to work under stress, we got stress and look what happened.
"So you need something that's robust, something that's fair, rather than a series of bolt-ons.
"In the UK, in Britain, our constitution is a series of bolt-ons and it's worked so far.
"But it looks to me that we're trying to bolt on on the fag end of a five-year Parliament things that just won't work."