THE UK Government has been told to get a grip on its "unravelling" defence budget after it emerged the costs of Britain's two new aircraft carriers are likely to soar by almost £2bn to 7bn.

Defence sources insisted the Coalition remained fully committed to seeing through the contracts, on which almost 5000 Scottish jobs depend, but it is thought one option for the cash-strapped Government could be to sell one of the carriers not in 2020 as mooted but in 2015.

Four years ago, when the project was announced, the initial costs were put at less than £4bn but delays led to them rising to £5.2bn as estimated in last autumn's defence review.

It is now thought the working assumption of the contractors – BAE Systems, Thales UK and Babcock – is that the final costs could be £7bn.

While the planes – the Joint Strike Fighter – due to go on the carriers will be cheaper, their conversion to use catapults and traps for take-off and landing, enabling British and French planes to land on each other's carriers, means the general cost will rise.

Downing Street said work on the defence review was continuing and "firm decisions" on final costs would be made late next year.

The plan is to have one of the carriers, HMS Queen Elizabeth or HMS Prince of Wales, fully operational with planes by 2020 when the other will be mothballed and possibly sold. No decision on which one will become fully operational has been taken yet but work has already begun on the Queen Elizabeth and work is due to begin shortly on the Prince of Wales, according to the Ministry of Defence.

Sections of the carriers are being constructed on the Clyde before they are taken to Rosyth and assembled in dry dock.

Yesterday, Liam Fox, the Defence Secretary, acknowledged costs would rise, saying: "If you want to fit the catapult and arrestor systems on to the carriers, that is going to cost extra money. If you decide to do it, it will take extra time."

He highlighted the flexibility of the Joint Strike Fighter and the "inter-operability" with the US and France of the "cat and trap" system, adding the military capability the changes gave Britain would "last for 50 years".

Last night, Jamie Webster, the GMB union convener at Govan, said he would be raising the increased costs issue at a regular meeting of the contractors due next Thursday.

He said: "The carriers is a big contract and not greatly loved by the Government. Defence is not sexy, defence is not popular, wars are costing the country money and that puts the squeeze on procurement.

"We are as nervous as anyone else in the industry but the carrier programme is going well and we're concentrating on that and on finishing the Type 45 destroyer contract," he added.

At Westminster, Jim Murphy, the Shadow Defence Secretary, argued the rise in estimated costs was more evidence the defence review had been badly thought through.

"Ministers must now be clear with the British public about the real costs of their decisions. This could be a £2bn bungle. The Government has totally mishandled the UK's carrier capability and now needs to be totally clear about whether both carriers will be completed and capable of deployment."

He added: "The defence review is unravelling and ministers must give the country confidence that they have a grip of the budget and equipment programme."

The row over the estimated increase in the carriers' costs comes just days after the resignation from BAE Systems of Alan Johnston, managing director of its maritime division. His role was overall charge of the yards at Govan and Scotstoun as well as the company's operations in England.

A company spokesman said Mr Johnston, 61, had asked to take early retirement and insisted his departure had nothing to do with the escalating carriers' costs.