Angela Brewer, 53, said she begged Thomas Hughes not to open fire after he suddenly pulled a gun from his trousers.

The Old Bailey heard that when Krystal Hart opened the front door of her home in Battersea, loner Hughes knocked her to the ground and shot her twice in the head at point-blank range. He was angry with her boyfriend.

"She didn't get a chance," said Miss Brewer. "He had her down and aimed the gun to her head.

"He pushed her down with his left arm. He said, 'Where is he?' She didn't get a chance to say anything.

"She had her hands on the top of her head and looked up. I couldn't do anything. He shot her."

Hughes, 40, was infatuated with Miss Brewer - described in court as "the neighbour from hell" - and became embroiled in her long-running feud with 22-year-old Ms Hart.

The women lived in flats in the same terrace house in Belleville Road.

On Good Friday last April, Hughes flew into a temper when Ms Hart's boyfriend David Siveter took down his car registration number.

Angie Brewer: Lived below Krystal Hart

Miss Brewer told the jury: "He phoned me to say he was coming round to sort this out. He sounded very agitated."

After he arrived, Miss Brewer said: "I was trying to say David went away in a car because I saw him.

"I was trying to protect him. But he just pulled it (the gun) out the back of his pants - the waist. He told me to get away."

Weeping, she said she had hoped Miss Hart would spot the commotion on her own CCTV camera and not open the door but it was eventually pulled ajar and Hughes pushed it open before firing a shot.

Miss Brewer tried to pull Hughes away. She said: "He was deranged. He was pointing the gun at me, at the stomach. I couldn't hang on to him any more. I thought, 'Oh s**t', this is going to get messy."

As she tried to lock her door behind her, she heard the second shot go off and dialled 999.

Jurors have heard that Miss Hart died almost instantly.

Hughes, of Battersea, denies murder and possession of a firearm with intent.

The trial continues.