Sir Alan West, who retired last year as First Sea Lord and head of the Navy, has warned that the costly demands of Iraq and Afghanistan might make cancellation of the carrier project attractive to a hard-pressed MoD.

Despite government statements that "urgent operational requirements" (UORs) such as new mine-protected vehicles and extra helicopters are met from Treasury contingency funds, this is not always the case.

The defence equipment budget, already overstretched by paying for Typhoon Eurofighters, Type 45 destroyers, Astute submarines and new Nimrod surveillance aircraft, is now being tapped to pay for unplanned kit for the Army's front lines. Six Merlin helicopters bought from Denmark in March to shore up battlefield mobility will cost £180m. All of that has to be taken from the equipment budget and will not be underwritten by the Treasury.

Insiders say the Treasury meets only 60% of many UORs from the contingency war chest, leaving the MoD to find the rest from its existing, overstretched budget. A review of defence spending priorities has been under way since last year and most senior officers say money is being funnelled away from the Navy and the RAF to support ground forces facing daily combat.

Sir Alan, who spent three years arguing the strategic case for two large aircraft carriers, said: "There are still those in the MoD challenging the requirement for their own parochial reasons.

"The pressures of the Comprehensive Spending Review make the large piece of equipment programme money that has not yet been committed highly attractive to those running much less important projects who can see a painful squeeze coming."

If the carriers were cancelled, Sir Alan believes the consequences would be far-reaching for decades to come and would endanger the UK's security. "I see the CVF project as a make-or-break challenge for the MoD and, indeed, a litmus test for the government's commitment to defence," he added.

Meanwhile, The Herald can reveal that the UK is delaying the in-service date of the US-designed F35 future strike fighters intended for the carrier force until at least 2017.

Britain was due to take delivery of 150 of the stealth aircraft from 2014 onwards in a £10bn deal.

The MoD is now facing a financial "black hole" as major bills become due for multi- billion pound projects in the middle of the next decade and is trying to delay deliveries to spread the load.