* Merkel insists treaty change needed to tackle euro crisis
* Cameron vows to enhance UK interests in any treaty change
* Sources say Merkel won't oppose stepped-up ECB bond-buying
* Energy exec says many CEOs working on post-euro scenarios
PARIS/BERLIN, Dec 2 (Reuters) - British Prime Minister David Cameron threatened on Friday to obstruct a Franco-German drive for swift change to the European Union's treaty, a sign of the difficulty leaders will face transforming Europe to to save the euro.
France and Germany are reaching a consensus that euro zone economies need to be bound more closely together if the single currency is to survive, which could mean changing the EU treaty to give Brussels powers to punish spendthrift euro states.
Austrian Chancellor Werner Faymann said there was a danger that the euro zone bloc would split up unless it implemented new rules and stuck to them.
"When we are not able to set up and keep to more conditions and ground rules, then many countries in the euro zone will no longer be able to pay the very high rates for sovereign bonds," he told the daily Krone.
"The next effect will be that you won't find anyone to buy them. Then the euro zone has to break up because of this.... it is a very real danger."
After talks with French President Nicolas Sarkozy, Cameron said he was not convinced treaty change was needed to reinforce the single currency zone, which Britain has refused to join. If the 27-nation bloc's charter were reopened at a crunch summit on Dec. 9, he would have his own agenda.
The British leader said euro zone institutions such as the European Central Bank needed to "get behind the currency" to convince markets that it had the required firepower, and member states had to make their economies more competitive.
"Neither of those things require treaty change, but if there is treaty change I will make sure that we further protect and enhance Britain's interests," he told reporters. There was no immediate comment from Sarkozy's office.
Cameron faces pressure from Eurosceptics in his Conservative party to loosen Britain's ties with the EU and secure guarantees that any move towards fiscal union on the continent does not harm the interests of the City of London financial centre.
Sarkozy tried to persuade him to allow stricter budget discipline procedures for the euro zone without insisting on returning powers over social and judicial affairs from Brussels to London or seeking a veto right over EU financial regulation.
German Chancellor Angela Merkel called earlier for rapid but limited treaty change to remedy what she sees as the root causes of Europe's raging sovereign debt crisis, warning that Europeans faced a "marathon" to regain lost credibility.
Outlining a long-term approach to tighter fiscal integration in the single currency area, with tougher budget discipline, she dismissed quick fixes such as massive U.S.-style money printing by the European Central Bank or issuing joint euro zone bonds.
"Resolving the sovereign debt crisis is a process, and this process will take years," Merkel told parliament, vowing to defend the euro, which she said was stronger than Germany's former deutschemark.
The chancellor travels to Paris on Monday to outline joint proposals with Sarkozy for treaty changes to create coercive powers to reject national budgets and impose automatic sanctions on serial deficit sinners. U.S. Treasury Secretary Timothy Geithner will meet key leaders and central bankers Dec. 6-8 in Europe the EU summit.
Next Friday's gathering is seen by some as make-or-break for the euro zone after a string of half-measures agreed too late by European leaders over nearly two years have failed to stop bond market contagion spreading from Greece to Ireland, Portugal and now Italy and Spain.
Sources close to Merkel said she was willing to see the ECB step up buying of troubled euro zone countries' bonds, alongside smart use of the bloc's rescue fund, as a bridging measure until budget controls took hold, but she did not see it as a lasting solution.
Her speech set the agenda for a week of intense diplomacy to try to frame a new political deal to restore market confidence and give the ECB grounds to act more decisively to defend the euro and support teetering banks.
The European Central Bank has been reluctant to commit to buying bonds in large quantities like the "quantitative easing" carried out by the U.S. Federal Reserve and the Bank of England.
ECB executive board member Juergen Stark said a solution was urgent but added finding it was the job of politicians.
"The lingering and expanding sovereign debt crisis must be halted to avoid macroeconomic and financial disaster, in the euro area and beyond," he said in a speech in New York. "No country is immune any more to a loss of market confidence in its public finances."
World stocks and European bonds continued to gain on hopes that euro zone leaders may be moving closer to a comprehensive solution to the debt crisis.
But in a sign that business leaders are beginning to doubt whether the currency will survive, the chief executive of Austrian energy group OMV said dozens of top European executives were working on post-euro contingency plans.
"I was recently in Paris with some other representatives of large companies and we discussed this question," Gerhard Roiss told reporters on Friday when asked if he had plans for a euro breakup. About half the 45 firms present had confirmed they were working on such scenarios.
ECB President Mario Draghi sent a crucial signal to markets on Thursday, opening the door to more aggressive action to help fight the euro zone's sovereign debt and banking crisis if governments adopted a new "fiscal compact".
Sarkozy embraced German calls for a new treaty tightening fiscal discipline in a policy speech on Thursday, but unlike Merkel he made no mention of greater powers for the European Commission and European Court of Justice.
Instead, the French leader, struggling to win re-election next May, called for an "intergovernmental" Europe in which the presidents and prime ministers of euro zone countries would be the ultimate arbiters over national budgets.
His socialist opponents denounced him for advocating an "austerity treaty" dictated by Germany. Merkel went out of her way to rebut such accusations, telling the Bundestag it was "misleading" to suggest Germans were trying to dominate Europe.
The president of the European Parliament, Jerzy Buzek of Poland, said treaty change could be "dangerous" because Europe's citizens were unlikely to warm to the idea.
EU diplomats said Paris and Berlin hoped to find agreement among all 27 member states for limited treaty amendments rather than having to take the more divisive route of drafting a separate blueprint for the 17 euro zone states or fewer.
German officials praised the conservative Sarkozy's courage in telling voters that France would have to overhaul its social model and cut public spending.
On the markets, German 10-year Bunds outperformed safe-haven U.S. Treasuries and British gilts as investors saw prospects of an EU summit deal and ECB action to ease funding for cash-starved banks and to counter a looming recession in Europe.
Italy's 10-year bond yield was down to 6.65 percent, well below the danger levels close to 8 percent they hit last week, which analysts said could make it impossible for Rome to refinance its debt next year. Spain's 10-year borrowing cost tumbled to 5.68 percent.
Sentiment has turned more positive since the world's major central banks took emergency joint action on Wednesday to provide cheaper dollar funding for European banks, a move which suggested they feared a funding crunch was imminent.
A key measure of dollar funding stress felt by euro zone banks, the three-month euro/dollar cross currency basis swap , has narrowed by 30 basis points since the coordinated central bank move to around minus 130 bps.