Shipping operators and ports were put on the alert last Friday while security chiefs investigated the threat. MI5 considered that the warning about a terrorist driving a lorry bomb on to a roll-on, roll-off ferry was “not credible” and ministers decided against closing ports.

However, port authorities were ordered by the Department for Transport’s special security division, known as Transec, to remain on “enhanced vigilance” after last weekend’s scare.

This alert came as the Home Office said that radiation-measuring devices had been installed at major ports to stop terrorists smuggling in radioactive material to make a “dirty bomb”.

Vehicles coming through ports believed to be at risk are now scanned by Geiger counters to uncover any nuclear threat as part of Project Cyclamen. The Government is also ready to send soldiers to all vital water establishments under emergency plans.

To calm public unease about terrorist threats, Downing Street had to intervene yesterday to say that it has no knowledge of any imminent attack. The Government took the unusual step of releasing the details of the security warning that it sent to ports. Ministers said that it showed that there was no “credible or specific” threat, only a warning for ports to stay on high alert.

Ferry operators said there was no drop in the number of day-trippers and haulage companies using British ports yesterday, but ferry travellers and freight companies appeared confused about mixed messages coming from Europe’s shipping authorities.

Dutch and German shipping authorities say they were told by their respective intelligence services that a lorry bomber was at large in Europe. Norwegian operators say that the alert was for an attack last Saturday, when a bomb hidden in a lorry would be detonated at sea. At the major ports in Britain yesterday, such as Hull, Felixstowe and Dover, officials said that they were told to check “outward bound” lorries trying to get on ferries. All buildings at Dover port were searched.

New security plans for shipping are due to be agreed next month at a meeting in London of the International Maritime Organisation.

Contingency plans are in place at key installations such as airports, mainline railway stations, underground networks, bridges and power stations, although operators concede that they can never be “terrorist-proof”.

The number of armed police officers at Heathrow is being substantially increased. Random passenger searches will also be increased.

The Government has given the Metropolitan Police an extra £46 million to recruit more officers for the fight against terrorism.

The London Underground says that it has emergency plans, but had not been given additional money for security measures. There were no gasmasks available, and no detectors in stations to warn of a biological or chemical attack.

Hospitals in all major cities have been designated to deal with any terrorist attack, with training to deal with chemical or nuclear assault.

Armed guards are to protect key water installations and treatment centres around the country in the event of a specific terrorist threat to the nation’s drinking water. A review of security throughout the water industry has led to increased security patrols at thousands of water premises throughout Britain.

Round-the-clock security staff are in place at important sites, while other premises are being routinely patrolled by guards. Sources at Water UK, the overall organisation for the industry, said yesterday that water supplies were being scrutinised.

Security chiefs warned the industry before the summer that terrorists linked to al-Qaeda might try to paralyse public services such as water and power supplies by electronic attack. For example, an electronic attack could sabotage the software governing the flow of water. The need for stricter security of such essential computer networks was sent by the National Infrastructure Security Co-ordination Centre, a little-known Whitehall unit responsible for national infrastructure.

Sources said, however, that it was extremely difficult to achieve mass contamination of water supplies because of the volume of water involved and the large quantities of poisonous chemical needed.

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