DAVID Cameron has been warned not to use his "pretty ill thought-out" plans for English Votes for English Laws to undermine the United Kingdom that so many people fought so hard for during the referendum campaign.

Alistair Darling, who headed Better Together, suggested English nationalism posed as profound a threat to the Union as Scottish nationalism did.

He confirmed he made a dawn phonecall to the Prime Minister on the morning after the September 18 vote, urging him not to give the SNP "another grievance, another reason to come in through the front door".

Accepting there was a legitimate question to be asked about how matters like English education or English health were dealt with at Westminster when so much of the same subject matter was devolved to Edinburgh, Cardiff and Belfast, the former Chancellor said conflating the issue of giving more powers to Scotland and Evel on the steps of Downing Street had been a "profound mistake".

He went on: "If I look at where we are now, I am concerned we get ourselves into a situation, where, in trying to answer the West Lothian Question...we undermine the economic and political union, which 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; nobody voted for that. There are some very profound issues here."

Mr Darling said he had looked at the proposals document put forward on Evel by the Tories and the Liberal Democrats and claimed the suggestions were "pretty ill thought-out; it raises an awful lot of very good questions and then proceeds to answer none of them".

Asked if Evel posed a threat to the Union, the Edinburgh MP said: "Almost inadvertently, I could see the Conservative Party climbing into the same bed as the SNP and ending up with a profound threat to that UK."

He explained other countries like Canada successfully dealt with handling different issues at different levels of government but accepted that there was a problem with creating a federal system here because England had so much of the UK's population.

"What I would rather us do is sit down, set out principles of fairness and(see) if this will work under stress...You need something that is robust and fair rather than a series of bolt-ons. In the UK, the constitution is a series of bolt-ons and it has worked so far but it looks to me we are trying to bolt on at the fag end of a five-year parliament things that just won't work.

"I just don't want to undermine the very strength of the United Kingdom that I and a majority of people in Scotland voted for," he added.

Mr Darling, who has been an MP for 27 years, made clear saving the Union was his greatest political achievement. He is due to step down at the General Election.