LONDON UNDERGROUND BLAST: POLICE RAID HOME LINKED TO SECOND MAN ARRESTED IN CONNECTION TO TERRORIST ATTACK

 Police search house in south-west London over Underground blast

British police have raided a home in south-west London linked to the arrest of the second suspect in a subway bombing on Friday that left 30 people injured.

Key points:

*Two men have now been arrested — a 21-year-old and an 18-year-old

*Police have raided two properties in neighbouring suburbs south-west of London

*The threat level is downgraded from critical to severe, meaning an attack is highly likely

A 21-year-old man was taken into custody just before midnight on Saturday (local time).

Investigators were on Sunday searching the home in Stanwell, about 25 kilometres south-west of central London, heading toward Heathrow Airport.

It came after police earlier arrested an 18-year-old man in the city of Dover — the port linking England to France — and raided a property in Sunbury, which neighbours Stanwell.

Residents in nearby properties in Sunbury were ordered to evacuate their homes for almost 10 hours before being allowed to return, with authorities establishing a 100-metre cordon around the building.

The attack at the Parsons Green underground station left 30 people injured when a homemade bomb exploded during peak hour.

Experts said the bomb — which was placed in a bucket and concealed in a plastic bag — could have been far more destructive if it had fully detonated.

Two days after the attack, all but one of the people injured had been released from hospital, Britain's National Health Service said.

One person was still being treated at Chelsea and Westminster Hospital, which has a special unit for treating burns.

Earlier, passengers were evacuated from a British Airways plane due to fly from Paris' Charles de Gaulle airport to London on Sunday morning (local time) after reports of a security threat, but authorities later determined it was a false alarm.

One passenger on the plane said it was surrounded by dozens of armed officers and firefighters as travellers were escorted off the plane.

James Anderson said the pilot initially told passengers there were technical issues, but later said "there had been a direct security threat involving our flight".

A spokesman for France's national gendarme service said police and firefighters rushed to the scene after receiving a security alert and that the plane was evacuated.

Each passenger and each bag was checked and the plane was thoroughly examined, but no threats were found, the spokesman said.

Police did not elaborate on the nature of the original alert.

"The safety and security of our customers and crew is always our top priority. Additional security checks are being carried out as a precaution," British Airways said in a statement.

On Sunday, Britain lowered its national security threat level to severe from critical after as the investigation progressed, Home Secretary Amber Rudd said.

"Following the attack on Parsons Green last Friday, the police have made good progress with what is an ongoing operation," she said.

"The Joint Terrorism Analysis Centre, which reviews the threat level the UK is under, has decided to lower that level from critical to severe."

On Friday, the UK threat level was raised to critical, meaning an attack was imminent — severe, the second highest level, means an attack is highly likely.

Meanwhile, Ms Rudd told the BBC it was too early to reach any final conclusions on how many people were involved in the attack, but said the UK had seen "a year like no other".

Britain has endured four other attacks this year, which have killed 36 people.

She also poured water on claims of responsibility for the attack by the Islamic State militant group.

"It is inevitable that the so-called Islamic State … will reach in and try and claim responsibility," she said.

"We have no evidence to suggest that yet, but as this unfolds … we will make sure we find out exactly how he was radicalised, if we can."

Hundreds of soldiers were deployed at strategic sites, including nuclear power plants and defence sites, in the wake of the attack to free up armed police to help in the investigation.