Gordon Brown announced an urgent review of NHS recruitment after news that all eight suspects arrested after failed car bomb attacks in London and Glasgow were either doctors or in related medical jobs.

The issue of terrorism hung heavily over Mr Brown's first appearance at Prime Minister's Questions, robbing it of much of its gladiatorial excitement as both sides cagily played the security card.

Mr Brown said that background checks on skilled migrant workers would be expanded, as would the worldwide “watch list” of potential terrorists to help warn other countries.

He added that there would be an "immediate review" of NHS recruitment after the discovery that a number of doctors had been arrested in connection with the bomb plot. The review would be carried out by Lord West, the former naval chief appointed as Mr Brown's new anti-terrorism minister.

Mr Brown also agreed to look at a request by David Cameron, the Tory leader, for a border police force, and stressed the need for cross-party co-operation on security matters. He confirmed that the Government was still committed to a Privy Council review of the possible use of telephone intercept evidence in terror cases that come to court.

Mr Brown said: “I hope right across the House, as right across the country, there can be unity in our determination to fight terrorism. I want to remind people just how brave and courageous were the explosive experts in London and those who tackled the terrorist activity at Glasgow airport.

He added: “It is vitally important the message is sent out to the rest of the world that we will stand strong, steadfast and united in the face of terror."

The news came as security experts relaxed the apparent terrorist threat to the UK from its highest level. Tonight, announcing that the level had been reduced from critical to severe, Jacqui Smith, the Home Secretary, said: "There is no intelligence to suggest that an attack is expected imminently."

However, she added: "The reduction of the threat level to severe does not mean the overall threat has gone away - there remains a serious and real threat against the United Kingdom and I would again ask that the public remain vigilant."

One phase of the fast-moving inquiry may be drawing to a close after a string of arrests in the UK and Australia, police sources said. Six people will continue to be questioned today at London’s high security Paddington Green police station. Among them is Sabeel Ahmed who was arrested in the Lime Street area of Liverpool on Saturday, police sources said.

Dr Ahmed studied at the Rajiv Gandhi University of Health Sciences, in Bangalore, the home city of Dr Mohammed Haneef . A seventh man remains critically ill with severe burns in a Glasgow hospital and an eighth, Dr Haneef, 27, is being questioned in Brisbane, Australia.

Two men who sparked a terrorism alert at a Blackburn industrial estate have been discounted from the inquiry after evidence was found of a cannabis factory.

A massive police investigation continues into Saturday’s car bomb attack on Glasgow Airport and the two failed car bombs found in the West End of London.

The majority of those held are linked to the NHS as doctors or trainee doctors while the only woman arrested is a trained laboratory technician.

Details of the link between the arrested people and the medical profession have sent shockwaves through the health industry.

Dr Hamish Meldrum, of the British Medical Association, said news that members of a caring profession may be involved was “appalling”.

One line of inquiry is that the two men who drove the flaming Jeep into the arrivals hall were the same people who left the Mercedes bombs in the capital.

Yesterday, Canon Andrew White, a British cleric working in Baghdad, claimed an alleged al-Qaeda leader from Iraq gave him a chilling warning of the attacks.

Canon White told The Times the terror leader warned him that this group was planning an attack and said “those who cure you will kill you”, an apparent reference to the NHS link.

One Whitehall source said today: “It has taken a while but it appears that there are some linkages which refer to some of the individuals that have been detained. “As a result of these linkages we have been able to assist the police in their investigations and to help speed up some of the investigations.”

Dame Pauline Neville-Jones, former head of the Joint Intelligence Committee, and Mr Cameron's new shadow security minister, said that she was shocked by the NHS link.

She told the BBC: “It sends rather a chill down the spine to think that people’s values can be so perverted. It means, obviously, that you can’t make any assumptions, or have any preconceptions about the kind of people who might become terrorists. It does mean that you widen the net, obviously.”

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