The second of the Royal Navy's next generation aircraft carriers is to be brought into service rather than sold off, David Cameron has announced.

The Prime Minister’s unexpected statement about the 65,000-ton HMS Prince Of Wales comes after Nato members agreed a pledge to maintain spending on defence at their two-day summit in South Wales.

The Prime Minister told a press conference the move was an "investment in British security", and would ensure that one carrier was always available “100 per cent of the time”.

However, he dodged a question about whether UK spending on defence might drop below the two per cent of GDP figure which is Nato’s target. At present, Britain is one of only four of Nato’s 28 members who meet the target but experts have warned that the UK defence budget could fall below it next year as spending cuts bite.

HMS Prince of Wales is being built along with HMS Queen Elizabeth in a £6.2bn project. There has been speculation that the HMS Prince Of Wales would be sold or mothballed as part of the drive to rein in spending. A final decision had not been expected until after next year’s general election.

Admiral Sir George Zambellas, the First Sea Lord, had previously increased the pressure on the Government by arguing that Britain's credibility "hinges on a carrier being available when the need arises".

He said:"Hope is not a reliable method of ensuring capability availability when a crisis erupts. That is why we need the effects of a UK carrier. It's the wrong moment to find out that nothing happens when you push the carrier button. So to ensure continuous carrier availability, that means having two carriers, not one."