* EU carbon permits end virtually flat
* Volume likely to dip next week
* EU-wide carbon registry to open in June
LONDON, April 27 (Reuters) - European Union carbon permits finished virtually flat on Friday, as market participants said they were uninspired by mixed energy prices and flagging demand ahead of a holiday in several countries next week.
The benchmark emissions allowance closed at 7.37 euros ($9.77) a tonne, a gain of 3 cents on the day and nearer to the top end of this week's 7.05-7.48 euro trading range.
Traded volume in the front-year contract on Friday reached a relatively thin 10 million units on ICE Futures Europe at 1700 GMT.
Carbon market participants held back from taking big positions due to a combination of mixed energy prices, a May 1 holiday in some European countries and limited demand for carbon permits from the power sector, traders said.
"There is no incentive for market participants to move in either direction," said a trader at an environmental commodities brokerage, adding volume is likely to decline next week.
"Some people will be on holiday and it will be the end of the compliance period," the trader said, referring to power utilities which generally need to buy CO2 permits to comply with the EU emissions trading scheme.
Energy prices were mixed. British gas prices defied bearish trading in the oil market, where Spain's sovereign credit rating downgrade weighed on investor confidence.
An EU-wide emissions registry will open in June, but accounts at existing EU member state registries will be inaccessible for two weeks prior to the open, the European Commission said on Friday.
As a result, none of the 30,000 registry account holders will be able to transfer emissions units during that period, meaning Europe's spot carbon market will effectively be closed. More details and the exact timetable will be released on May 3.
Traders said the temporary closure would have little impact on the market, because they could still buy and sell carbon permits on the forward market.
In the United Nations-backed carbon market, prices of certified emission reductions for December delivery slumped 1 percent to close at 3.81 euros a tonne.
CERs, which are issued to qualifying emission-reduction projects in developing countries, have fallen around 6 percent in the secondary market over the past week, as new supply weighs on limited demand.
Project developers have asked the U.N. to issue 7.5 million CERs next week, up 27 percent from the 5.9 million handed out this week, and above the 6.1 million weekly average seen so far this year.
($1 = 0.7542 euros) (Reporting by Jeff Coelho; editing by James Jukwey)