EU leaders were to hold their first formal talks on a draft constitution for the enlarged bloc, the top item on the agenda of a summit held amid high security in northern Greece. And they were warned not to unpick the blueprint, the work of 16 months of haggling between the 25 current and incoming EU member states. "This balanced result should not be lightly discarded or unbundled," said European Parliament President Pat Cox, at the start of the summit which kicked off Thursday in the northern Greek resort of Porto Carras. Former French president Valery Giscard d'Estaing was to brief the assembled leaders on the blueprint, aimed at preventing decision-making gridlock when the 15-member bloc expands to 25 members next year and perhaps more later on. Giscard d'Estaing, who finalized the draft last week after 16 months of haggling over the constitutional draft in the Convention on the Future of Europe, has urged EU leaders to change as little as possible.