The prime suspect in the disappearance of Madeleine McCann allegedly sneaked into a holiday flat and performed a sex act in a room full of sleeping British girls before boasting about it to pals.

A close pal of Christian Brueckner, 43, claims the German told him he had stripped naked outside first – and only fled when one of the teens woke up in terror and started to wake the others.

Brueckner is in a German jail suspected of kidnapping three-year-old Madeleine from a holiday flat in Praia da Luz, Portugal, in May 2007.

He is currently serving a 21-month jail sentence in Kiel, northern Germany, for drug offences after being extradited from Portugal to Germany in 2017 where he was jailed for sex offences against children.

Brueckner is now the prime suspect in the disappearance of Madeleine McCann

His friend has now broken his silence about the alleged sex act performed in a room full of sleeping girls after seeing this week’s police searches at an allotment Brueckner used to rent in Hanover.

They excavated the site, using sniffer dogs, ground penetrating radar and drones.

They apparently found a cellar under a stone slab and took away the concrete foundations of his old shack in skips.

He lived in the area then. His lawyer dismissed the searches as “pure desperation”.

German Police in Hanover at the search site in Hanover in connection with Christian Brueckner

His friend said: “We never thought his crimes involved children – he was a kleptomaniac. We just thought he stole everything in sight.

“He told me what he got up to when he lived in Portugal. He said he had been surviving by breaking into hotel rooms, stealing cameras and laptops.

“But the strangest thing he ever told me was about breaking into a room where four teenagers from Britain were sleeping on the sofa and floor.

“He was naked and had left his clothes outside and he was masturbating. When one woke up and tried to wake the others up he panicked and ran out.

Authorities found a 'secret cellar' at the allotment site

“He said he ran naked through the town. He told it like a funny story.”

The pal said he didn’t know for certain if this happened in Praia da Luz – but he believes it was in the same area.

He told how he befriended Brueckner at a kiosk the German owned in Braunschweig, North Germany, in 2013.

“We used to drink in a back room. He talked about his crimes and was kind of proud about it.”

Madeleine went missing while on holiday with her family in Praia Da Luz in the Algarve, Portugal

The friend said that Brueckner claimed he stole £100,000 from a house where the large deposit for a property sale was being stashed.

He claims Brueckner boasted of using the money to buy a house in Bavaria, where he lived as a child, and the kiosk. “It was all thefts, burglaries and drugs he talked about. He never mentioned children,” said the pal.

“He even claimed the palm trees in his garden were stolen. There was not one day when he didn’t steal something.”

The friend said the German claimed he also used some of his £100,000 haul to buy a disused factory in East Germany at an auction.

An extensive operation has taken place at the allotment site

Earlier this year police are said to have searched the grounds and found eight USB sticks in a bag, some containing child abuse images.

Brueckner ended up living at the Braunschweig garden allotment between 2013 and 2016.

Sabine Sellag, the owner of his former shack and plot, fears she may have been sleeping above the body of another missing child known as the "German Maddie".

Brueckner was quizzed but never charged after Inga Gehricke, five, vanished in 2015 near where he lived.

Police forces walk by the allotment

Locals said he prised open floorboards in the shack soon after arriving to gain access to an underground cellar.

He was one of around six people living on one of the 164 plots, which is forbidden.

“A few homeless people live there but they get away with it because it helps with security,” the pal said.

“He was friendly to people. I think he was lonely deep down.”

But after three years he was on the move again after police started to quiz him about missing Inga and accessing web porn.

“I think he went back to Portugal, I never saw him again,” said the friend. “He sold everything he had. He bought a car to leave with.”

Brueckner is now doing time for drug offences in a Kiel jail and is due out in January – but he has applied for an early release date.

He has previously denied involvement in the McCann case.

His lawyer Friedrich Fulscher has said he does not think he will be charged.

“As long as my client does not know what he is accused of, and on what basis, there is no reason to think otherwise,” he said.