THE bill for new giant aircraft carriers – being partly built at Birkenhead's Cammell Laird shipyard – has ballooned by a further £1bn, the Ministry of Defence admitted yesterday.

The escalating cost raised doubts about whether it was still cheaper to press ahead with the project, rather than cancel the second carrier – the decision taken reluctantly by ministers last year.

Meanwhile, one report suggested the real bill would be £7bn. That would put it nearly £2bn above the £5.2bn cost disclosed last autumn – and up from £3.9bn when the contract was originally signed, in July, 2008.

Cammell Laird shipyard is already building the flight deck and hangars for HMS Queen Elizabeth, under a £44m contract, and hopes to help build HMS Prince of Wales as well.

But HMS Queen Elizabeth will be put into "extended readiness" – effectively mothballed – three years after it enters service in 2019.

Behind the scenes, Government sources have suggested the carrier could be sold to another country, to recoup some of the cost of building.

Announcing the go-ahead last October, David Cameron condemned the "appalling legacy" left by Labour, which meant it was too late for the MoD to cancel the second carrier.