Image caption The douments detail the authorities' concerns about gas attacks

Germany planned to use chemical and biological attacks during a wartime invasion of Britain, according to documents from the National Archives.

Aircraft adapted to spray gas or foot-and-mouth disease, and even anthrax shells, were seen as possibilities.

The Nazis were "rapidly preparing" by March 1941 and would not hesitate to attack, say the papers.

Gas attacks on troops were viewed as most likely, but attacks on the public were also predicted.

The newly-released documents - from the Ministry of Home Security and other bodies - say the aim would be to cause panic and start an evacuation that would block roads and prevent defending forces reaching the coast.

Intelligence also revealed that the Germans were planning to falsely accuse Britain of using gas in order to justify its own attacks.

The documents detail the ominous movement of large amounts of chemicals from factories to "areas occupied by troops likely to take part in an invasion".

It also lists evidence from various secret witnesses and "observers".

They include a Swedish army officer who saw gas canisters ready to be loaded at German air bases, and a prisoner of war who "had volunteered information that his squadron had carried out tests near Vienna of gas spray".

'Cloud attack'

Intelligence chiefs were also mindful of a possible seaborne gas cloud threatening the country.

Germs of foot-and-mouth disease are reputed to have been sprayed, experimentally, from aircraft National Archive document

A document from August 1940 states that such an attack, using phosgene or chlorine, although "operationally difficult", could not be disregarded and could threaten an area up to 10 miles in width.

"Whenever an on-shore wind at night is forecast, a warning will be issued to all troops within five miles of the coast", reads the document.

The "warning zone" stretched from the east coast of Scotland all the way round the south coast of England to the Bristol Channel.

Alerting the public to a potential gas cloud "would be undesirable and should not be done" adds the secret memo.

'Biological warfare'

The possibility of anthrax and biological warfare is also briefly mentioned in the newly released papers, which date from 1939 to 1941.

"The germs of foot-and-mouth disease are reputed to have been sprayed, experimentally, from aircraft," says one report.

It continues: "Tests are said to have been carried out with shells infected with Anthrax... they are said to result in 95% mortality, death occurring in 10-12 days."

The archives also reveal that experts expected an initial surprise bombardment of up to 2,500 airborne gas sorties, delivered by bomb or spray, during the first few days of a Nazi invasion.

Image caption The British had more gas masks than the Germans say the documents

One document states: "The German High Command may think that, if gas is used on the first day... the resulting casualties and panic will enable them to get a firm foothold on these shores."

The planned German invasion, codenamed Operation Sea Lion, was postponed in September 1940 after the Luftwaffe's defeat by the RAF in the Battle of Britain.

Air superiority was considered vital for the Germans' amphibious assault on Britain.

Hitler eventually cancelled the invasion plans in January 1941.