Aircraft manufacturer Boeing could withdraw from the bidding for the £18.6 billion deal to supply the United States Air Force with new tankers unless the Pentagon changes the tender guidelines.

Such a move would leave the way clear for main rival Airbus manufacturer EADS and its US partner Northrop Grumman to take the prize, which is to supply the USAF with 179 new refuelling aircraft.

The consortium thought it had already captured the prize in February when it beat off Boeing only for the Pentagon to withdraw the offer after the US Government Accountability Office found significant errors in the selection process and upheld a Boeing protest.

The contract would have been a major to many aerospace companies in the UK, one being Black Country-based Hampson, which supplies materials for Airbus and its military variants.

Boeing has claimed that the revised deadline to prepare bids is unreasonable because the terms of the contract specify a plane larger than the 767 aircraft that had originally been proposed.

The Seattle company said it had not had sufficient time to prepare an alternative.

Loren Thompson, a US defence analyst, said the final decision on whether Boeing challenged the bidding rested with its chairman, however, she said that the dominant view within the company was that it had to protest because it could not win under the revised arrangements.

Other sources have said that Boeing has already invested too much time and money in the competition to pull out now.

Despite this, Ms Thompson said there was also a faction within Boeing that favoured withdrawing from the competition completely.

Boeing spokesman Dan Beck said the company was still evaluating its options.

“We have not made any decisions but all options are certainly on the table,” he said.

The Pentagon’s new tender suggests bidders will get credit for delivering more fuel than required.

The final draft is now expected next week and is likely to feature an aggressive schedule aimed at selecting a winner by the end of the year and putting an emphasis on fuel offload.

Boeing supporters say the new wording favours EADS/Northrop’s larger tanker based on the Airbus A330, and the current timetable is too brief for Boeing to prepare a bid based on a bigger aircraft.

Northrop says it was clear all along that the government wanted the most capability it could get at the best price.

Pentagon leaders insist they have only clarified their intent, not changed the requirements for the tanker, and that means the companies should be able to respond quickly.

Northrop and EADS are ready to respond and have urged a quick resolution of the issues.

If Boeing protests, the Pentagon could still require the companies to submit bids according to the prescribed schedule to keep the competition running, said one source familiar with the protest process.

Pentagon officials have already agreed to give Boeing and EADS/Northrop 60 days – two weeks longer than initially planned – to submit new bids once they receive the final draft next week.

However, analysts have said that even the additional time will not give Boeing enough time to prepare a bid based on the larger 777 airliner or the 767-400 version of the 767.

Boeing is leaning toward an early protest for several reasons, including a growing feeling that government officials are not taking company concerns seriously, added Ms Thompson.

She said some company officials had already wanted to protest against the original tendering, after some terms were changed to keep EADS/Northrop in the running.