The pilot is missing after the plane disappeared from radar on Tuesday night, 84 miles (135km) from the Misawa air base in the north of the country.
Wreckage has already been found after the incident - only the second crash in the plane's history.
Britain's RAF and Royal Navy currently have 17 F-35B jets in total, but the Ministry of Defence has pledged to buy 138.
The UK's stealth jet fleet was declared combat ready from land in January.
A MoD spokesperson said it was in close contact with US officials after the crash.
"Safety is of the utmost importance and very closely managed on the F-35 programme. We will continue to review the situation as further information becomes available," said the spokesperson..
Officials said the F-35A in the Japan crash was less than a year old and had done 280 hours in the air, while the pilot had 60 hours' experience with the plane.
It was at the front of a group of four aircraft on a training flight when it sent an "aborting practice" signal, said defence minister Takeshi Iwaya.
The jet was in the air for 28 minutes and flying in good weather when contact was lost.
Efforts to find the black box are expected to be difficult because the jet crashed in waters around 1,500m deep.
Japan said it was working with the US on determining the cause of the crash and the Pentagon is also monitoring the situation.
The high-tech design of the F-35 makes the 1,200mph jet "virtually undetectable" by enemy radar, according to its American maker Lockheed Martin.
The crashed plane was the fifth F-35 delivered to Japan but the first assembled inside the country, said an Air Self Defence Force official.
Mitsubishi Heavy Industries, which put the plane together at a plant near Nagoya, has so far not commented.
Japan's first F-35 squadron has only just become operational and the country plans to buy 87 of the jets to modernise its air defences as the military might of China grows.
The crash is only second involving an F-35 since it launched nearly 20 years ago.
An F-35B went down near a US Marine Corps base in South Carolina in September, prompting a temporary grounding of the aircraft.
The pilot in that incident survived after triggering his ejector seat.
A MoD spokesperson said it was in close contact with US officials after the crash.
"Safety is of the utmost importance and very closely managed on the F-35 programme. We will continue to review the situation as further information becomes available," said the spokesperson..
Officials said the F-35A in the Japan crash was less than a year old and had done 280 hours in the air, while the pilot had 60 hours' experience with the plane.
It was at the front of a group of four aircraft on a training flight when it sent an "aborting practice" signal, said defence minister Takeshi Iwaya.
The jet was in the air for 28 minutes and flying in good weather when contact was lost.
Efforts to find the black box are expected to be difficult because the jet crashed in waters around 1,500m deep.
Japan said it was working with the US on determining the cause of the crash and the Pentagon is also monitoring the situation.
The high-tech design of the F-35 makes the 1,200mph jet "virtually undetectable" by enemy radar, according to its American maker Lockheed Martin.
The crashed plane was the fifth F-35 delivered to Japan but the first assembled inside the country, said an Air Self Defence Force official.
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| Image:An F-35B crashed in Beaufort, South Carolina, in September |
Mitsubishi Heavy Industries, which put the plane together at a plant near Nagoya, has so far not commented.
Japan's first F-35 squadron has only just become operational and the country plans to buy 87 of the jets to modernise its air defences as the military might of China grows.
The crash is only second involving an F-35 since it launched nearly 20 years ago.
An F-35B went down near a US Marine Corps base in South Carolina in September, prompting a temporary grounding of the aircraft.
The pilot in that incident survived after triggering his ejector seat.
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