Image copyright PA Image caption HMS Prince of Wales will join its sister ship HMS Queen Elizabeth in active service

The Royal Navy's second new aircraft carrier, the Prince of Wales, is to be brought into service rather than sold off or mothballed, Prime Minister David Cameron has announced.

The news came as members of the 28-nation Nato alliance pledged to reverse declining trends in defence budgets at a summit in Wales.

The Prince of Wales' future had been in question since the 2010 defence review.

Work on the carrier, which began in 2011, is due to be completed in 2017.

Final assembly of the carrier should start later this year in Rosyth, with launch due in 2017.

The Prince of Wales' sister ship, HMS Queen Elizabeth, was officially named in July. Both carriers will not be fully operational until 2023, the Ministry of Defence said.

The £6.2bn cost of building two new carriers, first conceived in 1998, has led to speculation that one of them may be held in reserve or even sold upon completion.

But speaking at a news conference in Wales, Mr Cameron said bringing the Prince of Wales into active service meant the UK "will always have one carrier available, 100% of the time".

"They are an investment in British security, British prosperity and our place in the world, transforming our ability to project power globally, whether independently or with our allies."

He said the UK was one of only a handful of nations to spend more than 2% of national income on defence and would continue to have the second largest defence budget in Nato.