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The family of a London teenager missing in the searing heat of the Australian outback today made a desperate appeal for information.

Sam Woodhead, 18, was last seen on Tuesday evening after finishing work as a hand on a Queensland sheep ranch about 80 miles from the nearest town of Longreach.

He had apparently gone for either an evening or early morning run to avoid the daytime temperatures, which can soar to around 40C.

Sam, from Richmond, is on a backpacking gap year before joining the Army, where he hopes to train as an officer.

His family said today that they fear he may have been injured during his run.

His mother, Claire, 54, is en route to Australia to help with the search, in which a helicopter, light aircraft and large number of local volunteers mobilised .

The remote search area reportedly spans 135,000 acres around Upshot Station.

His sister, Rebecca, a model who is 21 today, told the Standard: “We’re on tenterhooks, we’re so far away — we don’t know what’s going on.

“They’ve called off the official search for tonight but my cousin has travelled from Hong Kong and chartered a helicopter with night vision to go up. There’s a couple more helicopters on standby.

“Sam left me a Facebook message on Monday, saying he would get on Skype for my birthday. He found it hard work being on the ranch, but enjoyed riding horses and quadbikes, he’s a fitness junkie.”

Speaking at the family home today, his father, Peter, 62, a retired intellectual property executive, said: “We’re all distraught. The next 48 hours are critical because we’re now into his third day out there. Injury is a possibility and the police have got no particular leads.”

The teenager’s backpack and running shoes were missing from his room, where he had been staying as part of his “outback experience”.

But other belongings, including his wallet, were still there.

Police said there was nothing untoward that would have caused him to just “take off” and reported he had gone running on previous occasions..

The alarm was raised on Wednesday morning when he failed to come down for breakfast.

His father said: “The lady who runs the farm gave him a call and found he wasn’t there.

“He will have gone running in the bare minimum, shorts and a t-shirt.

“We just hope he had some appropriate clothes, water and a torch in his rucksack.

“The police arrived at 5.30am yesterday to inform us and from then on we went into overdrive getting family members involved.”

Sam, a former Brighton College student, was due to return from travelling in the autumn ahead of applying for Sandhurst.

His father said he was inspired to join the Army by his grandfather, Second World War hero Lieutenant-Colonel Sam Derry.

He said: “After staying in Sydney and Brisbane, he wanted to work on a farm to use a segment of his time to do something that would stand him in reasonable stead to join the Army.”

Local police fear Mr Woodhead became “disorientated” during his run.

Sergeant Dave Perry said: “The owner of the property has gone to chase him because he wasn’t out of bed yet.

“He’s really into fitness. He jogged on a number of occasions.

“It’s possible he may have been jogging on this occasion and become disorientated.”

Police said that dehydration and snake bite were also concerns.