By John O'Donnell and Robin Emmott BRUSSELS (Reuters) - France and Germany led a group of countries on Tuesday in calling for a tax on financial trading, but their failure to agree central elements of the plan means it will fall far short of its original ambitions. The tax was promised in 2011 by German Chancellor Angela Merkel and then French president Nicolas Sarkozy as a means of getting banks to contribute more towards solving a crisis that had by then bankrupted Greece and Ireland. Finance ministers from ten euro zone states made a further pledge on Tuesday to phase in the tax on the trading of shares and some derivatives from January 2016, two years later than the original start date. But they left unanswered crucial questions such as how high the tax - opposed by a second group of European states including finance industry heavyweight Britain - should be and how it will be charged.