Britain's operator of gas pipelines and power cables is investing heavily in building new power lines to European neighbours like Belgium and manages Britain's gas and power exchanges with its neighbours on a daily basis.

Britain's new Conservative government has promised to hold a referendum on European Union membership by the end of 2017. National Grid, one of Britain's top 20 companies by market capitalisation, is convinced membership is vital for the British economy.

"We cannot afford to lose the access to (European) energy supplies and interconnection, whatever the framework is eventually," Chief Executive Steve Holliday told Reuters.

"Being part of the European energy market is unquestionably essential for the UK."

The company has started analysing how much value its exchanges with EU countries are adding to Britain and will publish its findings in a couple of months, Holliday said.

"When we've completed that piece of work we'll be using that to talk to government about those benefits," he added.

Many business leaders in Britain have said the country should remain a member of the EU.

National Grid reported an 11 percent rise in annual pretax profit on Thursday, allowing the company to pay shareholders a full-year dividend of 42.87 pence, up 2 percent year on year.

The utility made 2.9 billion pounds in pretax profit, ahead of a company-provided consensus forecast, and saw earnings per share rise 9 percent to 58.1 pence.

The company said it had invested around 3.5 billion pounds in improving its infrastructure in the financial year ending March 31 that would help reduce transmission costs and ultimately household energy bills.

Shares in National Grid were trading up 0.2 percent at 0706 GMT (8:06 a.m.).

(Editing by Mark Potter and Susan Thomas)

By Karolin Schaps