The chaos gripping Britain's aviation industry began to ease last night after the official terrorist threat level was lowered from its most critical rating.

Four days after police and MI5 smashed an alleged plot to blow up transatlantic passenger jets and stringent security checks were introduced, the threat level was reduced a notch from critical to severe.

That prompted a slight relaxation of the security regime at Britain's airports in a bid to ease the congestion endured by thousands of passengers since last Thursday's anti-terror raids.

However, the new regulations still leave Britain with one of the strictest pre-flight security routines in the world and much tighter constraints on the size of hand luggage.

Home Secretary John Reid and Transport Secretary Douglas Alexander said the measures were necessary because there was still a "very serious" risk of a terror attack, despite the downgraded threat level.

The severe rating meant an attack was still high likely, but no longer imminent, Mr Reid explained.

Detectives investigating the plot, in which the suspects allegedly planned to explode liquid-based explosive devices smuggled in hand luggage while in mid-flight, were continuing to question 23 people last night.

They were also conducting searches at several locations linked to the investigation, code-named Operation Overt.

Specialist officers were combing woodland at King's Wood in High Wycombe, Buckinghamshire, close to the scene of one of last Thursday's raids. However, Scotland Yard remained tight-lipped amid unconfirmed claims that police had found a rifle and a handgun and "suspicious" c omponents during the searches.

In Westminster, Community Secretary Ruth Kelly said the Government and Muslim communities both had to do more as they step up the "battle of hearts and minds" against Islamic extremism.

Speaking after more than three hours of talks with Muslim community leaders Ms Kelly acknowledged there were "different views" over aspects of Government policy.

But there had been a series of "sharp and challenging exchanges" which had been "very constructive and forward looking", she said.

At Heathrow airport last night, BAA's chief executive Tony Douglas said the airline industry had faced a "national security challenge" on an "unprecedented scale" in the past few days.

The new security regime will see: * All liquids banned, except prescription medicines and baby milk;

* The quantity of hand luggage slashed by about 50 per cent;

* Half of all passengers searched.

Mr Reid denied that the lowering of the threat level was based on "commercial" pressures and said the decisions were based on the analysis of independent intelligence experts, and not Ministers.

Mr Alexander said the new airline regulations incorporated the latest intelligence about terrorist capabilities, scientific and operational thinking. However, he added:

"We reserve absolutely the right to modify our security requirement as and when we become aware of new tactics and techniques which might be used against us."

In America, the country's Homeland Security Department reduced the threat level for US-bound flights from Britain from red, for severe, to orange, for high.

In Pakistan up to 17 others are believed to be in custody, including Briton Rashid Rauf, who is thought to be the brother of 22-year-old Tayib Rauf, one of those arrested in Birmingham last week.

The Home Office refused to confirm or deny whether the Government had formally requested his extradition.