Tube is high on the list of targets for terrorists

Eleven days after the Home Secretary accidentally disclosed that terrorists could strike Britain with deadly chemicals, fears have been raised that extremists may be planning to release poison gas in the London Underground.

Despite strong denials by security sources and the Government that there is a specific plot to hit the Tube with a gas bomb, the Underground has long been recognised as a likely terrorist target. It has always been considered vulnerable to bombs, gas, or fire.

And while there does not appear to be any intelligence to suggest that three north African men due to appear in court today on terrorist charges were involved in such a plan, MI5 and Scotland Yard's anti-terrorist branch are aware that supporters of al-Qa'ida could attack the Tube.

Concerns over the issue prompted the Government to reveal last week that it is planning a poster campaign advising people how to react to a chemical or gas attack.

The "protect and survive" posters will go up in Underground and mainline railway stations from early next year. Professor Sir Liam Donaldson, the Government's chief medical officer, said the campaign was being devised because of the increased risk of attacks by terrorists using gases such as sarin. He said that the advice could be "to leave the scene of the incident quickly, or not to take deep breaths".

Sir Liam said the increased threat of attacks, including the detonation of "dirty" (radioactive) bombs, meant people would have to become accustomed to seeing hospitals and emergency services practicing response plans. "It will be alarming for people to see exercises in which pretend casualties are carried away by people in protective equipment and decontaminated, but they will get used to it," he said.

The potentially devastating impact of a gas attack on an underground system was demonstrated on Tokyo's subway in 1995. Twelve people were killed and 5,000 were injured when the Aum Shinrikyo cult released sarin nerve gas.

Security has already been heightened at airports, ports, power plants, government and military buildings. The speculation about the Underground is almost certain to lead to a review of vulnerable parts of the country's transport system.

The alarm comes as British and American intelligence services have picked up an unprecedented level of intelligence suggesting al-Qa'ida is planning an attack some time soon. The main concern is the potential threat from suicide bombers.

Fears of an imminent attack prompted David Blunkett to issue a "threat assessment" document 11 days ago to alert people to the possible danger without alarming them. Officials mistakenly gave journalists an uncensored version which included speculation about different types of attack. It said terrorists might "try to develop a so-called dirty bomb, or some kind of poison gas; maybe they will try to use boats or trains rather than planes. The bottom line is that we simply cannot be sure".

Two days after the document was released, six men were arrested in London, including the three who are charged with terrorism offences. Police sources suggested the weekend reports linking the suspects to a specific plan to attack the Tube were journalistic invention. John Prescott, the Deputy Prime Minister, described it as press elaboration.

But there remains concern about a possible chemical attack and safety measures have been introduced, including the creation of a national stockpile of drugs and medical equipment to deal with anthrax, smallpox or exposure to radiation.

Emergency services have been overhauled. Hospitals will have 300 mobile decontamination units and 8,000 protective suits for use in terrorist attacks. Rapid response units are on stand-by to respond to emergencies or reports of suspicious packages. They include police officers trained in dealing with chemical, biological, radiological and nuclear attacks.

Japanese subway attack killed 12 with a Nazi nerve gas

A powerful nerve gas developed by the Nazis was used in the most deadly terror attack on an underground railway.

Seven years ago, members of a Japanese religious cult alerted the world to the horror of such attacks when they released sarin on the Tokyo subway. The Aum Shinrikyo members left small perforated bags of sarin in subway terminals so that the gas would seep out during the rush hour.

Twelve people were killed and about 5,000 people were treated for exposure to the gas. The army was called in to decontaminate five trains and 16 stations.

Also in 1995, there were several attempted cyanide attacks in Japanese underground stations. In one, a caretaker found a device that would have released cyanide gas at a lavatory in Kayabacho station, near the Tokyo Stock Exchange. No one was hurt. Weapons experts said there was enough cyanide to kill 14,000 people.

Exposure to high levels of cyanide as a gas, liquid or white powder can result in skin irritation, headaches, dizziness, loss of co-ordination, nausea, vomiting, gasping, increased blood pressure, loss of consciousness and death.

Cyanide smells like bitter almonds at dangerous levels. Earlier this month, Amaretto di Saronno called a halt to an advertising campaign in which its almond liqueur was wafted through the Tube, because of security fears that the smell was similar to cyanide.

Sarin is estimated to be 26 times more deadly. It is colourless, odourless, tasteless and diffuses very rapidly. The symptoms of sarin exposure include impaired vision, vomiting, headaches, drowsiness, convulsions and breathing difficulties. Sarin has been manufactured in Middle Eastern countries and was used during the Iran-Iraq war.