The Prime Minister and Scotland's First Minister today both welcomed the go-ahead for a £4.5bn investment in North Sea oil exploration.

The second phase of the giant Clair field, west of Shetland, forms part of £10 billion being spent on four projects by BP and its partners from Shell, ConocoPhillips and Chevron over the next five years.

At their peak, it is expected that the projects will provide 3,000 UK oil and gas supply jobs and play a part in sustaining the more than 3,500 jobs already existing in BP's North Sea operations.

David Cameron, visiting Aberdeen, and Alex Salmond both welcomed the go-ahead as proof that there was life in the North "for decades to come".

However, the First Minister repeated his call for the Treasury to revisit the additional tax imposed on offshore companies in this year's Budget.

The Prime Minister, who was at BP's headquarters today, said: "I am delighted to give the go-ahead for this project. This investment is great news for Aberdeen and the country and provides a massive boost for jobs and growth.

"It shows the confidence that there is to invest in the North Sea - we have cutting-edge technology, world-class skills and expertise and a UK Government that is committed to do what we can to secure future investment."

He added: "Aberdeen has been a destination for global investment for many years and I want to see that continue for decades to come. It continues to be at the forefront of new technology, new investment and world-class skills and expertise and shows a real can-do energy and drive to stay ahead and break new ground.

"The oil and gas industry is not only important for our energy security, but is a major source of jobs and is vital for future economic growth. There is still massive opportunity in the North Sea and I am determined to work closely with the industry here in Aberdeen to maximise this and do what we can to promote further investment and exploration."

Mr Salmond said: "This massive new investment by BP and its partners is extremely welcome and confirms that the offshore industry has a key role to play in generating jobs, skills and revenue for decades to come.

"With up to 40% of oil and gas reserves still to be extracted and well over half of the revenues still to be generated, the UK Government needs to give more certainty to the industry and restore confidence that has been badly dented by the Treasury's conduct this year.

"As today's announcement demonstrates, there is plenty of life left in the industry. Indeed, if it had not been for the Budget blow, it would be at the centre of an unprecedented boom in jobs and investment, not just in the west coast frontier area but in the marginal and brownfield places hardest hit by the tax hike.

"Concern remains over lost jobs and investment in the more challenging and mature fields and David Cameron should take the opportunity of his visit to Aberdeen to promise that at long last there will be a substantive response to the Scottish Government proposals sent to the Treasury in June, suggesting options to incentivise activity, particularly a rate-of-return allowance."

BP's £4bn involvement in the Clair Ridge represents the highest level of annual investment the company has made in the UK North Sea.

Chief executive Bob Dudley said: "Although it began over 40 years ago, the story of the North Sea oil industry has a long way yet to run. BP has produced some five billion barrels of oil and gas equivalent so far from the region and we believe we have the potential for over three billion more."

Earlier this year, BP and its partners announced plans for the £3 billion redevelopment of the Schiehallion and Loyal fields, west of Shetland, and the £700 million development of the Kinnoull field in the central North Sea.

Alongside development drilling and a number of smaller schemes, the four projects represent almost £10 billion of new project investment by BP and its partners into the UK continental shelf over the next five years.

Aberdeen-based BP operates around 40 oil and gas fields, four onshore terminals and a network of pipelines that transport almost half of the UK's oil and gas production.

Mr Dudley added: "After some years of decline, we now see the potential to maintain our production from the North Sea at around 200,000-250,000 barrels of oil equivalent a day until 2030. And we are working on projects that will take production from some of our largest fields out towards 2050."

Environment campaigners said the announcement showed an "utter contradiction" in the Government`s policies on energy.

Vicky Wyatt, from Greenpeace, said: "Nothing could highlight the utter contradiction of the coalition's position on energy and the protection of the environment more than David Cameron's dash north to announce more deepwater oil exploration off the west coast of Shetland.

"While Chris Huhne likes to portray himself as the good green guy of the Cabinet, all those around him are pledging the UK to a dirty-fuel future that will do only one thing: increase CO2 emissions and cause irreparable damage to the environment. It is now frankly risible for David Cameron to claim that this Government will be the greenest ever."