Moreover, he said, none of the documents reviewed so far carries a classification level above "secret" - the lowest category of intelligence material in terms of sensitivity.

A Northrop executive described the terms as a “lowest-common-denominator approach designed to favour a less capable, smaller aircraft by turning the contest into a cost shoot-out.”

The sharp rise in oil prices follows an equally sharp collapse over the previous two years, when prices fell by more than half to their lowest level in real terms since before the 1973 shock.