Since breaking out of HMP Wandsworth under a food delivery vehicle on Wednesday morning, Khalife will have been "bin raiding" or "shoplifting" to get food, survival psychologist Dr Sarita Robinson says.

Providing the suspected terrorist is still in London or has travelled to another large city, drinking water will also be easy to come by - through outdoor fountains or in fast food restaurants, she adds.

Daniel Khalife prison escape latest: Police investigate sighting of fugitive in Wandsworth and offer £20,000 reward

"Urban survival is challenging - but there's plenty of food and drink to be found if you can avoid the surveillance.

"You can survive for three weeks without food and three days without water - but he needs those things to keep his brain working."

With the whole country looking for him, the increased adrenaline he will experience could also cause him to make a mistake and unwittingly reveal his location.

Dr Robinson, of the University of Central Lancashire, tells Sky News: "Psychologically this is quite a challenge.

"That high level of adrenaline, being on hyper-alert all the time, and that increase in anxiety that goes alongside it… you can be more careless when you're under that stress and a simple slip can happen very easily."

Khalife, 21, is also likely to have a heightened level of testosterone, which could increase his propensity for violence, she says.

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Nick Yarris, an acquitted Death Row prisoner who survived for 25 days on the run in the US, agrees.

"Fear makes people do things they wouldn't otherwise do," he tells Sky News.

"I hurt people when I escaped. He's in a situation where he believes he could be gunned down at any moment. So he's in danger of harming someone in desperation."

Mr Yarris, who served time for a rape and murder he didn't commit, adds that with so much at stake, Khalife is unlikely to have slept since he went on the run.

"This is the biggest event of his young life. This isn't going to go the way he wants - he's going to make a huge mistake."

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Officials, including Met Police commissioner Sir Mark Rowley, have said Khalife's escape is likely to have been planned.

This could have been done by colluding with staff inside or accomplices outside the prison.

As well as the practical support of someone giving him access to a change of clothes, food, money, or shelter, having someone to help him may also give him the confidence to go on the run for longer, according to Dr Robinson.

"Having help would reduce his anxiety levels. That social support and reassurance would make slip-ups less likely," she says. "That's going to make it a bit harder for police to catch him generally because he'll have more options."

Hot weather likely to be a benefit - not a drawback

There are two other key elements that benefit Khalife, the experts say.

First, although the current hot weather may be seen as a disadvantage, warmer night time temperatures mean he won't have to seek cover or warmth after dark - keeping him away from areas with CCTV or people who might spot him.

"It's sticky and uncomfortable in the day, but it's not so hot that there's a danger to life," Dr Robinson says.

"As long as he keeps hydrated, the fact that it's staying in the 20Cs (68F) in the evening means he can keep warm at night.

"We have people who are homeless for example - they manage day-to-day, even if it's uncomfortable."

Khalife's military background will also be invaluable, Mr Yarris says.

"Even at introductory level in the military - you at least know how to get into the woods."

The police's decision to search Richmond Park in southwest London demonstrates that "propensity for going into nature", he adds.

And if he's able to steal a bike or an e-scooter to get further afield, it could lend itself to camping out on a farm or inside a houseboat, he says.

'Mental toughness' from military could also help

Retired military officer Major Michael Shearer agrees, noting that even in his first of 14 weeks' training he will have "started to learn how to live independently but among friends".

Major Shearer says that during his four years at Beacon Barracks in Staffordshire he is unlikely to have had specific training in escape and evasion - but he will be better placed than most people to "handle pipe pressures" - like going to war or being on the run.

Dr Robinson agrees that the "mental toughness" offered by even a basic military rank will "help you understand your own limitations when under pressure".

Asked whether he's left the country, Mr Yarris says enhanced checks at airports and ports make that a virtual impossibility.

But Major Shearer notes that "we've got thousands of illegal immigrants that our Border Force cannot manage".

"How we find one individual on the way out - I'm at a loss to comment on," he says.

Mr Yarris adds that in Khalife's shoes he would have travelled to "another major metropolitan area" such as Birmingham or Manchester to "hide any other crimes I'd have to commit for sustenance".

Police have since said there was a confirmed sighting of Khalife in southwest London on Wednesday morning.

An eyewitness reported seeing a man fitting Khalife's description "walking away from a BidFood van that had stopped near the south entrance to Wandsworth Roundabout", about a mile from Wandsworth Prison.