Two men and a woman have admitted dumping mustard gas in a lake near Woodhall Spa.
Martyn Tasker, 40, his wife Michaela Tasker, 31, and Stuart Holmes, 50, appeared at Lincoln Crown Court where they admitted a breach of the Chemical Weapons Act by possessing mustard gas canisters.
The charge relates to dates between September 27 and October 1, 2017.
The three also admitted breaching environmental protection laws by causing the discharge of mustard gas into Stixwould Lake near Woodhall Spa, on September 30, 2017.
The charges follow the discovery of 150 mustard gas canisters on the site of a former military base at Roughton Woods, near Woodhall Spa, which led police to declare a major incident.
Specialists from Porton Down were brought in by the Ministry of Defence and the area was sealed off for a fortnight.
The subsequent clean-up operation cost almost £300,000.
The canisters are believed to have been unearthed by bottle diggers searching the woods for vintage bottles. Two people were later treated in hospital for minor burns.
Martyn and Michaela Tasker, 31, both of Longdales Road, Lincoln, and Holmes, of Witham Road, Woodhall Spa, each entered guilty pleas to both charges.
Holmes also admitted disposing of mustard gas in a manner likely to cause pollution to the environment or harm to public health between September 28 and October 1, 2017.
Martyn Tasker denied two charges of possession of a Bren light machine gun and a further charge of possession of 10 rounds of ammunition without a firearms certificate on October 4, 2017.
He is to face trial on those charges on March 3 next year.
Judge John Pini QC ordered pre-sentence reports to be prepared on Michaela Tasker and Stuart Holmes.
They will be sentenced along with Martyn Tasker following the conclusion of Martyn Tasker's trial for the firearms offences.
What is mustard gas
Poison gas was used to devastating effect by both sides during the First World War, resulting in 1.3m casualties.
Mustard gas causes temporary blindness, breathing problems, and stomach upset.
The use of chemical and poison weapons was banned under the 1925 Geneva Protocol but production and storage was never outlawed.
It is said that Britain had planned to use such weapons against the Nazis had they invaded these shores, and that RAF Woodhall Spa would have dropped chemical weapons on Berlin had the Germans dropped gas bombs on London.
