(Adds US Marine Corps statement)
LONDON, July 9 (Reuters) - British Defence Secretary Philip Hammond said Lockheed Martin Corp's F-35 fighter jet may miss the Royal International Air Tattoo air show this week given a fleet-wide grounding ordered after an engine failure last month.
But Hammond told a conference in London on Wednesday that he was "still optimistic" that the new warplane would be cleared to fly in time to make a delayed international debut at the world's biggest air show in Farnborough, outside London, next week.
Military and industry officials are pressing hard to ensure that the jets are able to make their international debut at the Farnborough show. Missing both shows would be a huge embarrassment and could undermine U.S. efforts to drum up additional sales of the new fighter.
U.S. Air Force and Pentagon officials at the conference had no immediate update on the safety investigation into the engine failure that occurred on June 23 as a pilot was preparing to take off from Eglin Air Force Base in Florida.
The U.S. Marine Corps still plans to send some of its F-35B jets - which can take off from shorter runways and land vertically - to Britain this month, spokesman Captain Richard Ulsh said.
Engine maker Pratt & Whitney, a unit of United Technologies Corp, is examining the engine at its West Palm Beach facility in Florida, but has not released any details about the likely cause of the engine failure.
The engine incident has already forced the F-35 to skip a planned "fly by" at the naming ceremony for Britain's new aircraft carrier last week, and U.S. industry and military officials are not certain whether the grounding will be lifted in time for the planes to travel to the British air shows.
Hammond said Britain, which helped fund development of the new warplane, had a "huge commitment" to the F-35 program. He noted that the plane was still in the developmental stage, when technical issues are meant to be worked out.
The program has completed about 60 percent of developmental testing, according to the Pentagon's F-35 program office.
Hammond said U.S. officials made the "right decision" when they suspended all F-35 flights while authorities investigated an engine failure and fire that occurred on June 23 when an F-35A was preparing to take off from a Florida air base.
"At this stage of the program we can afford to take a cautious approach," he told a conference hosted by the Royal United Services Institute for Defence and Security Studies.
"I don't have concerns about the overall program," he said.
He said the program was "close to running out of time" to fly F-35 jets to Britain in time for the RIAT air show that starts in Fairford later this week, but he remained upbeat about the jets appearing at the Farnborough air show next week.
Hammond said the UK defence budget agreed for the rest of the parliament was tight but was manageable. Efficiency savings could still be made but further cuts would be damaging, he said.
Britain has cut defence spending by around 8 percent over the last four years as part of a government plan to reduce a record budget deficit, leaving it with an army which by 2020 will be the smallest it has been since the Napoleonic Wars of the early nineteenth century.
"It is clear to me that we couldn't take further significant reductions in the defence budget without it having some very meaningful consequences," he said. (Additional reporting by Kylie MacLellan; Editing by Mark Potter and Louise Heavens)