The inquest into the MI6 agent’s death heard how a number of experts unsuccessfully tried to lock themselves inside a bag up to 300 times.

But Jim Fetherstonhaugh, 49 – a sergeant in the Royal Artillery for 22 years – discovered a simple method by which a holdall can be zipped up and locked from the inside without assistance.

The technique is the reverse of a well-known trick used by airport thieves to steal from locked bags, as long as they are flexible and the sides can be pinched together.

Intrigued by the mysterious case of the MI6 agent, he asked his daughter Izzy, 16, to climb inside an identical North Face bag to test his theory.

The teenager, who at 5ft 5ins was 3ins shorter than Gareth, easily fitted inside the bag in the foetal position before partially closing it.

She drew her legs up to her body and was able to draw the two zip pulls together while leaving a gap for her hands to protrude through and close the padlock.

Izzy then tensed her body and the zip simply sealed itself leaving her successfully shut inside the bag holding the key to the padlock.

But crucially, the procedure is said to be much easier in a bath where the occupant of the bag can push against the sides for assistance.

Jim, of Shrewton, Wilts., served around the world as a sergeant with the 33 Regiment, Royal Artillery, for 22 years until he retired five years ago and has been a reservist ever since.

He claims that, despite Mr Williams being taller than his daughter, this would not have made much difference.

He said: "The man on TV tried it 300 times and I was amazed he couldn’t do it.

"I’ve told my friends and show them and their jaws drops – it is so obvious, maybe people are thinking too much into it.

"She is 5ft 5 and he was 5ft 8 but when you’re in the foetal position inside that height doesn’t make much difference.

"She had a bit of a struggle pulling the bag but she wouldn’t have been as strong as him. I could easily see that he would be able to do it.

"She did it on the floor and it would have been much easier for him in the bath because he could put pressure on the sides to manoeuvre himself in.

"Once you show people how you do it, their jaws drop. I wonder if the services do know this method – but they don’t want it known.”

A similar technique is used to steal from locked holdalls, where the end of the bag can be squeezed together to create ‘slack’ in the zip.

This creates enough room for an opening in the zip without unlocking the padlocks.

Mr Fetherstonhaugh contacted the Met Police via the force’s 101 number and he was provided with a serial number but they have yet to respond to him.

A spokesman for the force refused to comment yesterday (Weds).

The revelation raises questions over the coroner’s findings, which concluded Mr Williams’ death "unnatural and likely to have been criminally mediated”.

Mr Williams’ former landlords told the inquest how they once found him handcuffed alone to a bed, suggesting he had a fetish for restraint.

* To test the method, SWNS reporter Claire Hayhurst, who is 5′ 4” tall and of slight build similar to Mr Williams, zipped herself into the bag without assistance.