Whitehall security officials are suggesting that key sites such as the Milford Haven gas terminal on the Welsh coast and national grid power stations that supply London and other cities should be protected by police armed with machineguns.

The move follows intelligence reports in September that at least two known Al-Qaeda suspects had been spotted reconnoitring a huge gas pipeline facility at Bacton on the Norfolk coast.

Bacton is made up of five separate terminals and handles about a quarter of the country’s gas, processing supplies from three big North Sea fields. It also sends and receives gas to Zeebrugge in Belgium and the rest of Europe through its interconnector terminal. Effectively Britain’s “gas supply tap”, its huge underwater pipelines are critical to the country’s economic wellbeing.

A security source said: “Police received intelligence of reconnaissance of the Bacton terminal by suspected Al-Qaeda terrorists. The suspects were sufficiently well traced [on intelligence files] for it to be worrying.”

Officials say proposals to create an armed force to protect the critical national infrastructure have been discussed in the Cabinet Office and the Department of Trade and Industry (DTI). They fear that infrastructure sites are relatively poorly defended, often only by unarmed security staff.

A successful attack could have devastating consequences, blacking out entire cities and cutting off water supplies.

The new force is likely to be created by extending the existing Civil Nuclear Constabulary (CNC), a dedicated police force that protects more than 40 nuclear power plants and other civil nuclear sites across Britain.That force deploys several hundred officers, many of them armed, at sensitive nuclear sites such as Sizewell in Suffolk and Sellafield in Cumbria.

Senior officers from the force are understood to have held meetings in the past month with Cabinet Office and DTI officials to sort out details of the plan.

Patrick Mercer, the Tory spokesman on homeland security, said: “We have long argued for the need for our energy sources to be properly protected. While it’s taken the government an inordinately long time to do something about it, at least something is now being done. I shall be interested to see how long it takes them to put these very sensible plans into practice.”

Al-Qaeda has focused on using suicide bombers to attack so-called “soft targets” such as the London Underground, shopping centres, nightclubs and stadiums. Now Islamic terrorists are believed to have added strategic economic sites to their list.

Eliza Manningham-Buller, the director-general of MI5, hinted at the change in a speech 10 days ago, in which she highlighted the growing terror threat. “We are aware of numerous plots to kill people and to damage our economy,” she warned.

The Home Office, which is responsible for MI5, declined to comment on the threat to Bacton or other economic targets.

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