It is understood that finance ministers and officials need more time to work out the details of what will be agreed by euro zone heads of government at their summit meeting in Brussels. Euro zone leaders are expected to announce plans to boost the firepower of the EU's bail out mechanism - the EFSF - through the combined use of two schemes. Under one, the issue of sovereign bonds will be partly insured, while a second scheme will involve the International Monetary Fund, through a so-called Special Purpose Vehicle. Leaders are also expected to announce the losses which are to be imposed on holders of Greek sovereign debt as part of the renegotiation of Greece's second rescue package. The losses will be much deeper than those announced at a summit on July 21. The debt write down is expected to be between 50% and 60% compared with a 21% haircut last July. The insurance option - known as a credit enhancement - means that the EFSF will be able to insure between 20% and 30% of a bond issue as a way to keep borrowing costs down for countries like Spain and Italy. It will be complemented by an SPV which will allow the IMF to mobilise the resources of private investors, sovereign wealth funds and emerging economies like China and Brazil. Officials are said to be working out the legalities of the two schemes and trying to avoid pitfalls. One such problem could be that insuring a portion of a debt issue could be seen as preferential treatment for a particular class of bondholders at the expense of other categories. Another problem could be that a country which avails of the credit enhancement could find that availing of the scheme could technically add to its debt level. The EU's statistics arm Eurostat is understood to be examining the implications of the mechanism.