Royal Navy and Army bomb disposal teams have so far cleared a total of 150 mustard gas canisters from Linconshire.
According to the BBC, the Ministry of Defence will also remove contaminated soil from sites where a large quantity of canisters were found.
It came after two people were treated in hospital when they discovered canisters containing the gas on 1st October.
They were taken to the Defence Science and Technology Laboratory at Porton Down, Wiltshire.
The canisters were believed to date back to the site's use as an RAF base.
But the RAF base there (RAF Woodhall Spa) was built in 1941, and mustard gas was last used by the British military in 1919...
So what gives?
Well, while neither Germany nor the Allied nations used toxic gases intentionally in combat during World War 2, they did stockpile the weapons.
Britain had plans to use mustard gas on its beaches in the event of an Axis invasion in 1940 and was ready to put it to use on the battlefield if the Nazis did first.
The Geneva Convention prohibited the use of poison gas post-WW1 but didn’t prevent its manufacture and storage.
Chemical weapons recovered by Royal Navy divers in Lincoln.
The RAF was tasked with being the main deliverer of the gas, with nine Blenheim squadrons and five Army Co-operation squadrons ready to pour it on invading soldiers as they landed on British beaches.
The Army also had around 40,000 gas shells stockpiled and ready to use.
In total, it’s thought that Britain produced 40,719 tonnes of mustard gas between 1939 and 1945.
Much of it ended up being buried – like the bombs found in Lincolnshire.
Eran Bauer a local defence analyst and historian was contacted by the police to research the chemical weapons in Lincolnshire.
He collated the information, providing police with information to act on within hours.
Mr Bauer said: “The police asked if I knew about a 6lb round bomb – I said I did. I said: ‘I’ll send you everything I have’”
“They were manufactured in the early 40s. It was a hand-thrown grenade – lit like a pyrotechnic.”
6lb Round Bomb
The small 6lb hand-thrown weapons found at Woodhall Spa were to be used as area denial weapons to create a toxic cloud in a limited area.
They are tin canisters with a pull-off metal cap, underneath which is a pyrotechnic match head and striker.
Despite the chemicals being found on a former RAF base, Mr Bauer said the weapons were not to be used by the RAF.
There is a clear warning printed inside the 1940 handbook defining the use and deployment of chemical munitions which says:
"the British Government has no intention of initiating the use of gas".
RAF Woodhall Spa did have huge chemical bombs which were to be dropped on Berlin in the event of the Nazis dropping chemicals on London.
“These bombs were professionally removed at the end of the war. They were the size of a mini, can you imagine digging a hole to dump that in?”
Woodhall Spa was also used as a training area for the Army with small-arms ranges and troop accommodation.
Woodhall Moor Camp was the largest of three camps, Mr Bauer accessed a 1947 RAF aerial photograph from archives to enable the police and military teams to understand the area.
Mr Bauer said: “It’s likely that the hand-thrown bombs found recently were leftovers from that use."
“Most of it was properly disposed of at the end of the war.
"But I think someone was probably lazy and told some squaddies to get rid of it, so they just dug a hole and chucked it in.
“Can you imagine at the end of the war, after D-Day, everyone was about to demob, it’s the last thing they want to do [cleaning up chemical weapons].”
What Is Mustard Gas?
Toxic gas is a weapon used almost exclusively in the First World War.
Most gases were produced and used by Germany (more than 68,000 tonnes), followed by France and then Britain (more than 25,000 tonnes).
Mustard gas was the most widely reported of many poisonous gases and was thought to be the most effective of the First World War.
It was introduced in 1917 and saw use by Germany, Austria and the Allies.
The chemical was not officially categorised as ‘lethal’ but, if inhaled, can cause death from bronchial pneumonia – burning the respiratory system.
Other symptoms include: blistering/burning skin and blindness (usually temporary).
Aerial photo of a chemical attack in the First World War.
How Many Chemical Weapons Are Left In The UK?
The finds in Lincolnshire have highlighted a problem that has been bubbling since the end of the Second World War.
Mr Bauer said: "No one knows how many [chemical weapons] there could be around the UK.
“There must be more around the UK, never mind this lot.
"This goes back to 1946 and it’s as live as it was 70 years ago.
“It just tells you how dangerous chemical weapons are even 70 years later.”
An effort to quantify the number leftover chemicals weapons was set up by the MOD in 2007.
Named Project Cleansweep, it investigated 14 former chemical weapon sites and found that they were all "generally suitable for their current use".
The bombs found on the site of RAF Woodhall Spa highlight that there are chemical weapons out there that have not been accounted for.