A defence contract due to bring thousands of jobs to the North-East could be delayed by two years because the Ministry of Defence "moved the goal posts," it was claimed last night.

A warning from lead contractor BAE Systems that the Royal Navy must find an extra £1bn or accept a sharp reduction in the size of its two new aircraft carriers could push the project back to the drawing board.

And MoD bosses have been blamed for making a series of revisions to the original designs, making the original £2.9bn budget impossible to meet.

A senior industry source said last night: "A worst case scenario is that the project will have to go right back to the drawing board, putting the project back between 18 months and two years. It could involve a massive amount of design work."

The security of more than 5,000 jobs in the North depends on the contract to build the biggest carriers since the Second World War.

Last night unions representing shipyard workers in the North called for a swift resolution to the problem.

Richard Scott, naval editor of Jane's Defence Weekly, said: "The MoD's aspirations do not appear to be in line with the budget.

"What they are experiencing now is a reality check regarding what can be achieved for the money they have available."

And a defence analyst in the region said: "This is mission creep of the first order.

"The specification of the carriers has got out of hand with bits and pieces being added continually to the original design along the way.

"The MoD has been hit with the price tag and it has frightened them."

BAE was awarded the contract in January but was ordered to work jointly with French contractor Thales.

Concerns have been raised about the divisions of work and the problems of two former rivals working together.

Swan Hunter is in line to carry out a third of the work from 2004 - which is expected to last ten years - and is preparing to double its workforce to 3,000.

Swan's Wallsend shipyard is expected to recruit subcontractors throughout the region, creating a further 2,000 positions.

The reported warning by BAE that if the cost of the project is not to reach £4bn, the size of the carriers may have to be reduced from 65,000 tonnes to 40,000 tonnes - cutting the capacity from 45 aircraft to 20 - is unlikely to cut the jobs created in the region.

But Jimmy Skivington, an officer of the GMB union which represents many of the workers at Swan Hunter, said: "We hope common sense prevails and this is resolved quickly. The North-East needs these jobs as soon as possible."

Norman Brownell, Swan Hunter commercial director, said: "We are in no way alarmed by this report but at this stage it is too early to say if it will have any impact at all.

"If a smaller ship was the outcome, then at the most it would mean pushing back the commencement date. But ultimately skill and expertise will still be required to build it."

And a spokeswoman for BAE moved to play down the row. She said: "The MoD set out their general requirements, and the money available.

"We are at the assessment stage where we work with the customer before coming to a final agreement."

The MoD denied that it was looking to change to a "cut-down" carrier force, but confirmed that it was looking at a "wide range of options" in the current assessment phase, ahead of the award of the final manufacturing contract next year.

"We are not looking to change to a cut-down carrier," an MoD spokesman said. "Various options are being investigated as a routine part of this process and nothing will be fixed until the major investment decision is taken next year with the award of the demonstration and manufacturing contract."

The ships - currently codenamed the Future Command Vessels - are planned to go into service in 2012 and replace HMS Invincible, and the Tyne-built HMS Illustrious and HMS Ark Royal. Fabrication work will take place in a number of locations - including Tyneside - before final assembly goes ahead in Rosyth, Scotland.

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