MI5 has lowered its assessment of the risk of a terrorist attack against Britain for the first time since the July 7 bombings.

The decision was taken to reduce the threat from “critical” — the highest rating — to “severe” last Thursday, government officials disclosed last night.

It indicates that the intelligence services do not have any specific information relating to imminent attacks on Britain.

Despite the change an official stressed that protective security measures are still in place. The level of alert, which determines how buildings and transport systems are guarded, remains unchanged.

The decision to change the threat level was taken by the Joint Terrorism Analysis Centre, which makes the assessment independently of ministers and reports directly to MI5’s director general, Elizabeth Manningham-Buller.

The Conservative spokesman on homeland security criticised the decision to downgrade the threat level secretly. “The police have said their biggest asset in tracking down terrorists is the public — it is important that they understand the procedures.”

A Home Office spokesman said last night: “It is our policy not to comment on the threat level for security reasons. There are regular reviews of the level of threat.”

Meanwhile, the government has admitted that nearly 200 people arrested as suspected terrorists are asylum seekers given safe haven in Britain.

Home Office records show that 182 — or one in four — of the 717 men and women detained as terror suspects since 2001 have claimed asylum in the UK.

While their applications are processed, all are entitled to state benefits, such as free housing and legal aid to pursue claims that they would be persecuted in their home countries.

The Home Office releases its latest asylum figures this week and the disclosure will re-ignite the debate over the country’s “porous borders”.

Sir Andrew Green, the former British ambassador to Saudi Arabia and chairman of MigrationWatch, the independent think tank, said: “This is clear evidence of the abuse of the asylum system by potential terrorists.

“We need a much tougher and more realistic approach to applications from countries which we know to be potential sources of terrorism.”

A number of those charged in a plot to target Britain with the deadly poison ricin were asylum seekers, including Algerian-born Kamel Bourgass, the gang’s ring leader.

Abu Qatada, a radical Muslim cleric said to be Al-Qaeda’s “spiritual ambassador” in Europe, came to Britain as an asylum seeker from Pakistan in 1993.

It has also emerged that two men charged over the July 21 attacks on London had come to Britain as dependants of asylum seekers from Eritrea and Somalia.

The Home Office says many of the 717 suspects detained have now been released. Few were charged with terrorist offences and only three have been convicted, although more than 20 are awaiting trial.

A separate Sunday Times investigation has revealed that Britain’s most influential Muslim leader holds shares in an internet company that has helped to run a British-based website with links to Al-Qaeda.

Sir Iqbal Sacranie, the secretary general of the Muslim Council of Britain (MCB), has a stake in Webstar, a firm that has hosted a website for Saad al-Fagih, a Saudi extremist living in London.

Al-Fagih is the head of a dissident group called Movement for Islamic Reform in Arabia (MIRA), accused by the American government of providing a platform for Al-Qaeda. The MIRA website has posted messages from Osama Bin Laden and Abu Musab al- Zarqawi, Iraq’s most wanted terrorist.

An MCB spokesman said: “Sacranie is a minority shareholder, with about 2,000 shares in Webstar. He had absolutely no idea Webstar was hosting this website.”

Al-Fagih denies any connection with terrorism.

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