The two new carriers are due to enter service in 2014 and 2016
Work to build two giant aircraft carriers may be affected by the economic downturn.
Work is scheduled to begin next spring in Glasgow on HMS Queen Elizabeth and HMS Prince of Wales.
But the Ministry of Defence has confirmed all its major programmes are under cost-cutting scrutiny.
Union leaders said they had not been told of any delay to the contract, but said it could be that it would be extended over a longer period of time.
The two warships would be the biggest ever built in the UK. They are to be constructed on the Clyde and at Rosyth in Fife, as well as at yards in Portsmouth and Barrow-in-Furness.
They are due to enter service in 2014 and 2016, but it has been reported that fears were growing in the shipyards of a potential "slippage" in the contract.
The defence ministry said: "The MoD is looking at all its major equipment programmes over the next 10 years, with a view to bearing down on costs and making sure the frontline troops are being properly supported.
"We hope this work will conclude soon, but at the moment no decisions have been made."
The green light for the new warships - which would create or underpin a total of 10,000 jobs - was given in May by the then defence secretary, Des Browne.
Contracts worth about £3bn were signed in July on board HMS Ark Royal, the flagship of the fleet.
The contracts included £1.325bn for giant sections of both ships to be built in Glasgow and Portsmouth, £300m for the construction of giant sections at Barrow, and £675m for the bow section and final assembly of both ships at Rosyth.