Ukip leader Nigel Farage has demanded a federal state (REUTERS/Dylan Martinez)

David Cameron has moved quickly to answer the constitutional crisis he created with the panic-stricken vow of new powers to Scotland towards the end of the referendum campaign.

But, into the bargain, he may also have unleashed a different strain of nationalism that has largely lain dormant – and Ukip's Nigel Farage cannot believe his luck.

The very moment the three Westminster leaders unveiled their vow, aimed at saving the Union from the apparently advancing Scottish nationalists, Tory backbenchers started shouting "what about England?".

Facing the threat of a serious backlash that has already seen a number of his MPs refusing to back his promise to Scotland, he came up with the only available answer. Devolution, he said, must be extended to the entire UK.

"We have heard the voice of Scotland - and now the millions of voices of England must not go ignored," Cameron said.

"So, just as Scotland will vote separately in the Scottish parliament on their issues of tax, spending and welfare, so too England, as well as Wales and Northern Ireland, should be able to vote on these issues - and all this must take place in tandem with, and at the same pace as, the settlement for Scotland."

Ed Miliband
Ed Miliband is pressing for constitutional change (Reuters)

That was enough to shock many in Westminster. But his pledge to sort out within weeks all the long-standing constitutional anomalies that the UK parliament has lived with for decades knocked the breath out of them.

And it gave the green light to all those on his own backbenches who have been pressing for an English parliament or executive.

And it led to instant demands from Wales and some of the big cities for their share of the pie.

Cameron, and Ed Miliband alongside him, undoubtedly mean well and the two genuinely want to re-balance the constitution to avoid any resentments at the special treatment handed to Scotland.

But they must beware. The anti-politics, anti-Westminster, anti-establishment mood gripping many voters, and played on by Alex Salmond, has also been Farage's strongest card and his version of little England politics is already threatening to make common cause with Tory right-wingers.

He will make hay with the "what about the English" sentiment now being encouraged by the prime minister. He has already demanded a full constitutional convention to hammer out a new federal UK and for Scottish MPs to promise not to vote on English legislation.

There is, as Miliband has said, a real appetite for fundamental change in the UK where the current ways of doing things are no longer up to the job.

If Cameron and Miliband can genuinely address that and forge a new deal that unites and engages citizens across the UK, rather than dividing or pushing regions into competition with each other, they may indeed pull off something historic and positive.

But the simple fact this is all supposed to be done in a few weeks and was clearly a last-minute, cobbled-together panic measure doesn't bode well.

And if they mess it up and succeed only in setting regions and nations against each other in the squabble over cash and power, only one person – or more accurately one political force – will benefit.