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Renewable energy sources must become "financially sustainable" if they are to meet the world's needs, David Cameron has said.

Opening a meeting of international ministers later, he hailed progress made by the UK in the "green energy revolution".

But the prime minister said the challenge now is to drive down costs.

He also welcomed the investment of £350 million on energy projects that will create 800 jobs.

Taking office in 2010, Mr Cameron pledged to lead the "greenest government ever", but environmental campaigners say that promise is so far proving hollow.

Ministers from 23 countries gathered in London on Thursday for a clean energy summit.

Opening the event, the prime minister said the world urgently needs "a more diverse, cleaner mix of energy sources that will give us... security without causing irreparable damage to the planet".

"Renewables are now the fastest growing energy source on the planet. And I am proud that Britain has played a leading role at the forefront of this... revolution," he added.

"Britain has gone from virtually no capacity for renewables, to seeing them provide almost 10 per cent of our total electricity needs last year. And we've added more capacity... in the last two years than at any time in the last decade."

He said that the UK now had "a different challenge. We need to make it financially sustainable."

The prime minister stressed the need for governments and businesses around the world to work together to lower the cost of renewable energy and develop a global carbon price.

He said that between April 2011 and February 2012 investments worth £4.7 billion have been announced in UK renewable energy and its supply chain, supporting or creating 15,000 jobs.

He also welcomed confirmation of further investments, including a new contract for British firm Balfour Beatty to upgrade the Humber Gateway offshore wind farm, and a £300 million biomass project by Helius Energy at the Port of Bristol.

Friends of the Earth executive director Andy Atkins said: "In the very week we find the UK back in recession there is more need than ever to build a clean economy, and create the thousands of jobs this would bring.

"With soaring gas costs sending household fuel bills rocketing, David Cameron should be championing clean British energy and leading the drive for a low-carbon future, not dodging the chance to advance this on the world stage."

Mr Cameron also announced a new initiative bringing together firms with an interest in maximising the potential for renewables in the North Sea.

The move comes after Chancellor George Osborne faced criticism from green groups for announcing £3 billion of new tax breaks for offshore fossil fuel extraction in his March Budget.