A British scientist wears a protective suit in a laboratory. MI5 has warned that al-Qaida is seeking to recruit scientists who develop deadly viruses. Photograph: Alamy

Dozens of suspected terrorists have attempted to infiltrate Britain's top laboratories in order to develop weapons of mass destruction, such as biological and nuclear devices, during the past year.

The security services, MI5 and MI6, have intercepted up to 100 potential terrorists posing as postgraduate students who they believe tried accessing laboratories to gain the materials and expertise needed to create chemical, biological, radiological and nuclear weapons, the government has confirmed.

It follows warnings from MI5 to the Foreign and Commonwealth Office that al-Qaeda's terror network is actively seeking to recruit scientists and university students with access to laboratories containing deadly viruses and weapons technology.

Extensive background checks from the security services, using a new vetting scheme, have led to the rejection of overseas students who were believed to be intent on developing weapons of mass destruction. A Foreign Office spokesman said the students had been denied clearance to study in the UK under powers 'to stop the spread of knowledge and skills that could be used in the proliferation of weapons of mass destruction and their means of delivery'.

He added: 'There is empirical evidence of a problem with postgraduate students becoming weapons proliferators.' The overseas students, a number of whom are thought to be from 'countries of concern' such as Iran and Pakistan, were intercepted under the Academic Technology Approval Scheme, introduced by universities and the security services last November.

The findings raise questions over how many terrorist suspects may have already infiltrated the UK's laboratory network. Rihab Taha, dubbed 'Dr Germ', who worked on Saddam Hussein's biological weapons programme, studied for her PhD in plant toxins at East Anglia University's School of Biological Sciences in Norwich.

In addition, a number of well-educated Iraqi scientists - funded by Baghdad - infiltrated several British microbiology laboratories in the run-up to the Gulf war of 1990-91. Britain has about 800 laboratories in hospitals, universities and private firms where staff have access to lethal viruses such as Ebola, polio and avian flu or could acquire the technology and expertise to develop deadly weapons. Whitehall sources remain concerned about the number of countries intent on acquiring the materials and knowledge to develop a nuclear or biological warfare capability.

John Wood of the National Institute for Biological Standards and Control said: 'Any scientist would say it's important that we know who is working in our laboratories, and also why they are working there.'

The trial of two NHS doctors, Mohammad Asha, 27, a Jordanian national, and Bilal Abdulla, 29, from Iraq, who allegedly plotted widespread carnage through car bomb attacks in London's West End and Glasgow airport last year, has intensified scrutiny on the radicalisation of students. Named in the plot is 27-year-old Indian PhD student, Kafeel Ahmed, who drove a Jeep laden with gas canisters into Glasgow Airport's main terminal building but died several weeks later from severe burns. Ahmed studied for his PhD in the technology department of Anglia Ruskin University in Cambridge.

A spokesman for Universities UK, which represents vice-chancellors, said the security scheme had so far proved effective. He added: 'It is important to protect the UK from people who may wish to use technology and materials here inappropriately.'

Michael Stephens, head of security at the Medical Research Council, which runs some of Britain's most sensitive laboratories, said they took the issue of biosecurity 'extremely seriously'.

Concern that al-Qaeda is intent on developing a more sophisticated weapons capability moved the former director-general of MI5, Dame Eliza Manningham-Buller, to warn publicly that terror attacks in Britain could involve weapons of mass destruction. She said: 'We know that attempts to gather materials are there, we know that attempts to gather technologies are there.'

Extremist groups are known to have targeted students, offering to fund courses in return for using their newly acquired expertise. It is unclear if any of those denied 'clearance' to study in the UK during the past year were funded by grants from host governments such as Tehran.

A Foreign Office spokesman said 'efforts' on scrutiny of foreign postgraduate students would continue with only a few of the 20,000 applications rejected for security reasons. In the US, draft legislation advocates banning all non-Americans from laboratories which possess potentially dangerous bacteria and viruses, a measure the UK government believes is too draconian.

Professor George Griffin, chairman of the advisory committee on dangerous pathogens, has warned of the lack of a national standard required for people to work in high-security laboratories.

The move comes as the government considers plans to build a new pathogen research facility in central London, between King's Cross station and the British Library. Experts have warned that a terror attack would prove catastrophic to the surrounding area.