Detectives investigating the blast in Stanley Road, Harrow, north London, on May 7 think it was caused by a gas leak, Scotland Yard said.

Police had linked the explosion to a substance poured through the letterbox of number 21 but this has now been ruled out as a cause.

Detective Chief Inspector Colin Sutton said there was a gas supply to No 21 which had fallen out of use. It appeared "a large volume of gas" passed through the meter on the day of the blast, he said.

Charlotte Anderson, 17, who lived at the address, suffered severe burns in the explosion, which killed Emmad Qureshi, 26, and left his friend, who has not been named, with head injuries.

Mr Sutton said police entered the property on Wednesday and removed the gas meter, doormat and piping for forensic examination.

He said officers had spoken to a group of young girls seen outside the house on the morning of the blast and alleged to have poured a purple liquid through the letterbox.

He said: "None of the information gathered in the past few days supports the theory of explosives, or similar devices, nor a deliberate act to cause an explosion.

"Although there was no gas appliance in use at number 21, there was a supply - and a meter - which had been unused for some years.

"Preliminary indications are that a large volume of gas may have passed through the meter on May 7, and it is this which has now become the focus of our investigation which we are conducting hand in hand with the Health and Safety Executive."