The US Government auditor has found plans to build a fighter jet which will be Australia's main combat aircraft are massively over-budget and face long delays.

The Government Accountability Office says the Joint Strike Fighter project could come in more than $40 billion over budget.

Problems in its development could also delay the completion of the planes by more than two years.

The F35 Joint Strike Fighter is planned to become Australia's main fighter jet from 2014.

Australia has ordered 100 of the planes at a cost of at least $14 billion.

The Joint Strike Fighters are also being sold to Britain, Canada and the Netherlands.