An immediate review of NHS recruitment will be carried out following the attempted bomb attacks in London and Glasgow, Gordon Brown says.
A Jeep Cherokee was driven into the main doors of Glasgow Airport
The review comes after eight suspects with links to the health service were arrested. Two are brothers.
Background checks on skilled migrant workers will be expanded, said the prime minister.
Meanwhile, security experts are considering relaxing the terror threat to the UK from "critical" to "severe".
Mr Brown told his first question time as prime minister it was "vitally important that 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 new terrorism minister Sir Alan West will carry out the NHS review.
Mr Brown said sponsors of skilled workers would be asked to provide background checks on them.
The Home Office was unable to provide more details about how the new checks would work.
NHS Employers, which represents health trusts, said on Tuesday that the NHS checked applicants' clinical and linguistic ability rather than their political or extremist affiliations and did not think that this should be changed.
Among other measures, Mr Brown also said a watch list of potential suspects would be expanded to warn authorities across the world, and the admissibility of intercept evidence in court would be reviewed.
New agreements will be signed with countries around the world to ensure a co-ordinated response to the terror threat.
A burning green Jeep Cherokee loaded with gas cylinders crashed into the doors of Glasgow Airport's on Saturday afternoon.
The previous day two Mercedes containing petrol, gas cylinders and nails were found in central London.
30 June Dr Kafeel Ahmed and Dr Bilal Abdulla arrested at Glasgow airport after burning car driven into doors of main terminal
30 June Dr Mohammed Asha, 26, and his wife Dana Asha, 27, arrested on the M6 near Sandbach, Cheshire
30 June/1 July Dr Sabeel Ahmed, 26, arrested near Liverpool's Lime Street station
1 July A 28-year-old man and a 25-year-old man, thought to be medical students or doctors from Saudi Arabia, arrested in Paisley
2 July Dr Mohammed Haneef, 27, detained in Australia, and a second doctor is questioned
3 July Second doctor questioned in Australia is released without charge
Seven of those arrested are believed to be doctors or medical students and one used to work as a laboratory technician.
Six are being questioned at London's Paddington Green police station.
A seventh, Dr Kafeel Ahmed remains in hospital after the Glasgow attack, and an eighth, Dr Mohammed Haneef, is still being questioned in Australia after his arrest at Brisbane Airport.
Dr Kafeel Ahmed is the brother of Dr Sabeel Ahmed, who was arrested in Liverpool on Saturday.
The first phase of the investigation - rounding up suspects to prevent further attacks - is drawing to a close.
Investigators are instead focusing on analysing evidence and interviewing suspects.
If the Joint Terrorism Analysis Centre concludes another attack is no longer imminent, the threat level will return to severe.
That would mean a slight reduction in security procedures and controls.
Dame Pauline Neville-Jones, former head of the Joint Intelligence Committee, and new shadow security minister, said she was shocked by the NHS link.
"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."
Security sources have told the BBC none of the suspects had been under surveillance or the subject of an anti-terrorism operation before.
But details of some of them were on an MI5 intelligence database because of their alleged links with other individuals or inquiries.
Canon Andrew White, a British cleric working in Baghdad, claimed an alleged al-Qaida leader in Iraq warned of the attacks.
Mr White said the man warned his group was planning an attack and said "those who cure you will kill you", an apparent reference to the NHS link.
Meanwhile, passengers at Heathrow Airport suffered further disruption on Wednesday as thousands of travellers tried to re-book flights cancelled following a security alert.
Queues formed early outside Terminal 4 as passengers who were turned away on Tuesday after a suspicious package was discovered tried to continue their journey.
More than 100 flights were cancelled on Tuesday.
Police have urged anyone with information to phone the confidential Anti-Terrorism hotline number on 0800789321.