The RAF's ability to continue operations over Libya into this autumn has been called into question by a senior officer.
Air Chief Marshal Sir Simon Bryant warned that the RAF was "running hot" over its contributions to the western support of Libyan rebels fighting renegade leader Muammar Gaddafi.
Both equipment and personnel were being placed under "huge demand", he wrote in a briefing paper for MPs seen by the Telegraph newspaper.
"Should Operation Ellamy (Libya) endure past defence planning assumptions the future contingent capability is likely to be eroded," he wrote.
His warning will be of serious concern to David Cameron, who has pledged to maintain pressure against Colonel Gaddafi until he abandons his grip on power in Tripoli. Western intervention has prevented the Benghazi-based rebels opposing him from being overrun, but not tipped the balance decisively in their favour.
"The true strength is in our people in continuing to deliver, despite all that's asked of them," Sir Simon added.
"Morale remains fragile. Although fighting spirit remains positive, this assessment will be challenged by individual harmony targets as Operation Ellamy endures (after September)."
He flagged up "decreasing satisfaction" with wages and said the strategic defence and security review (SDSR) was continuing to "undermine the sense of being valued".
Sir Simon added: "There is concern over the perceived lack of strategic direction which is restricting confidence in the senior leadership."
David Cameron said that his discussions with military chiefs had reassured, not concerned him, however.
"They are absolutely clear we are able to keep up this mission for as long as is necessary and that time is on our side, not on Gaddafi's side," he told journalists at a Downing Street press conference.
He pointed out Britain has the backing of the UN, Nato and many Arab League countries before adding: "The pressure is turning up all the time.
"I'm absolutely confident we can maintain this mission for as long as is necessary. We're growing in strength in terms of the transitional national council in Benghazi and we'll keep working with them to make sure we bring this to a happy conclusion."
Sir Simon's warning comes a week after First Sea Lord Admiral Mark Stanhope said naval forces would be have to be 'reprioritised' if the Libya mission continues for longer than six months.
Labour repeated its calls for the SDSR to be reopened, suggesting defence cuts were at the root of the problems the RAF faces in the skies over Libya.
"The assumptions, on which the government's rushed defence review were based, were fundamentally flawed," shadow defence secretary Jim Murphy said.
"It is time to listen to military advice, review the review and provide our forces with capabilities which match our foreign policy ambitions.
"This Tory-led government's decision to cut too far and too fast must not threaten Operation Ellamy, our standing commitments or our ability to project force overseas in future."