Cameron risks backlash by uncorking Britain's 'constitutional genie'

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Scotland's "No" to independence may have saved Prime Minister David Cameron his job, but sweeping pledges of a constitutional shake-up could undermine his re-election drive and trigger more political instability.

Responding to what he called a "clear" rejection of Scottish independence on Friday, Cameron, who is up for re-election in May 2015, promised to begin a process that would see Scotland granted further powers.

He also said he wanted to see more powers devolved to Wales and Northern Ireland, as well as changes for England, starting with new voting arrangements in the British parliament.

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Some, including in his own party, feel he promised too much.

"This result presents both opportunities and challenges for Cameron," said Matthew Ashton, a politics lecturer at Nottingham Trent University.

New Scottish powers must be fair for England: MP

"On the one hand he can make claim to the title of 'the man who saved the union'. On the other, he'll now have to deliver on his extraordinary ambitious promises of a new constitutional settlement."

In the closing phase of the referendum campaign, Cameron and other party leaders made detailed promises to Scotland, about future funding and new tax and spending powers - a move some of his own MPs described as a "panicky" response to opinion polls which suggested the vote was too close to call.

It will be difficult for him to renege.

"The genie of a more devolved UK can't be put back in the bottle," a senior source in the Lib Dems, Cameron's coalition partner, told Reuters after Cameron set out his plans. "The world has changed."

Read More UK politicians eye 'looser federation' after Scotland vote

Cameron, who might have been cast aside by his party as the leader who lost Scotland had the vote gone the other way, said the constitutional changes should be agreed as a package by the main political parties before the next election, so that they could be implemented in the next 2015-2020 parliamentary term.

With Scotland being given more say over its own affairs, Cameron says Mps from England - which comprises 83 percent of the British population - should also have a way to decide issues for themselves.

That might mean setting up a system to keep Scottish members of the British parliament from voting on UK policies that do not apply to self-ruling Scots.

Such a system could be good for Cameron's Conservatives, whose base is in England and who have been all but wiped out in Scotland. But it is tricky for the opposition Labour Party, which relies on Scottish support in Westminster and whose last UK prime minister, Gordon Brown, was a Scot.

It is hard to see a quick consensus emerging within months among major parties about a reform that could cost a future Labour government the power to pass laws through an English caucus potentially controlled by the Conservatives.

Read More The rebound from 'No' vote will be short-lived

Labour leader Ed Miliband said plans for change needed to be put to members of the public through a constitutional convention, rather than be "fixed solely by politicians or prime ministers trying to shore up their position in their own party".

Labour proposed the constitutional convention for autumn 2015 - after the next general election - which Cameron's Conservatives said amounted to kicking the issue "into the long grass".

Across the political spectrum, politicians said Cameron had opened up a question that will be hard to solve quickly because of partisan political differences and could hand an electoral gift to populists such as the anti-EU UK Independence Party (UKIP). Such wrangling also risks political paralysis.

Failure to deliver greater powers to English MPs could leave Cameron vulnerable to the electoral threat of UKIP, whose leader Nigel Farage promised to champion English voters.

"Crumbs of change"

So far, much of the criticism has come from Cameron's own MPs, who have already begun to complain that his promises to Scots are too generous and his pledges to England too limited and hard to deliver.

"The chaotic manner in which the 'No' vote was won has undermined the strong and strong and resilient United Kingdom on which we all depend," Owen Paterson, a Conservative lawmaker and a former minister, said in a statement.

"Such a lopsided constitutional settlement cannot last; it is already causing real anger across England. If not resolved fairly for all the constituent parts of the UK for the long term, it will fall apart."

Conservative lawmaker Andrew Percy said Cameron's proposal to allow only parliamentarians from English constituencies to vote on English matters was welcome, but wasn't enough.

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"Now it's England's turn for a say and we won't settle with being fobbed off with a few crumbs of change," he said. "That means a proper conversation about an English Parliament, English Executive and English First Minister."

Boris Johnson, the mayor of London and a potential successor to Cameron as leader of the Conservative party, said Cameron's promise to maintain Scotland's current funding deal - which means Scots get substantially more government money spent on them per head than the English - was "slightly reckless" and shouldn't be honoured in its current form.

Rob Wood, Chief UK Economist at investment bank Berenberg, said in a research note there was "likely to be a backlash from MPs (members of parliament) against particularly Prime Minister Cameron for guaranteeing generous funding and more powers for Scotland without consulting parliament.

"The same may be true for non-Scottish voters. That brings risks with it," he said.