US Congress is under growing pressure to examine why a £20 billion contract to supply aerial refuelling tankers to the US Air Force was awarded to Europe's EADS instead of Boeing, the American aerospace group.

Both the Democratic and Republican presidential candidates have also waded into the political row that has escalated since the contract was awarded to EADS, parent company of Airbus, and its US partner, Northrop Grumann, last Friday.

Nancy Pelosi, speaker of the House of Representatives, said yesterday that the deal, which was announced on Friday, posed "serious questions that Congress must examine thoroughly".

She said: "Given the ramifications of this decision for the United States, the Air Force must explain to Congress how it meets the long-term needs of our military and the American people."

Senator Hillary Clinton, who is battling against Barack Obama for the Democratic presidential nomination, said: "I am deeply concerned about the Bush administration's decision to outsource the production of refuelling tankers."

She added that it was "troubling" that the Government had chosen to award the deal to a "team that includes a European firm that our Government is simultaneously suing at the WTO for receiving illegal subsidies".

EADS is currently the subject of a lawsuit over claims it received $100 million in subsidies that allowed it to undercut its American rivals on large contracts.

Senator Obama also expressed disappointment that an American company had not won the contract.

Ms Pelosi questioned whether due consideration had been given to the number of jobs that would have been provided if Boeing had won the contract.

But Senator John McCain, the frontrunner to become the Republican presidential candidate, said: "I've never believed that defence programmes, that the major reason for them should be to create jobs."

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